Photovoltaic power system and control method thereof

ABSTRACT

A photovoltaic power system includes a DC/AC inverter circuit, N DC/DC converter circuits that are located at a previous stage of the DC/AC inverter circuit and that are respectively connected to photovoltaic strings, and a controller connected to the DC/AC inverter circuit and the N DC/DC converter circuits. The controller is configured to: perform MPPT control on n DC/DC converter circuits, and perform CPG control on (N−n) DC/DC converter circuits. In the two control manners, a fast and accurate power reserve or limit of a photovoltaic string inverter with any illumination intensity and ambient temperature can be implemented, and fluctuation of a DC bus voltage and AC output power of the photovoltaic string inverter can be eliminated. Control on a virtual synchronous generator of the photovoltaic string inverter is implemented, and a lifespan of the photovoltaic string inverter is prolonged, without a need to add an energy storage element.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/CN2019/088343, filed on May 24, 2019, which claims priority toChinese Patent Application No. 201811354438.4, filed on Nov. 14, 2018.The disclosures of the aforementioned applications are herebyincorporated by reference in their entireties.

TECHNICAL FIELD

This application relates to the field of photovoltaic power generationtechnologies, and in particular, to a photovoltaic power system and acontrol method thereof.

BACKGROUND

A photovoltaic power system mainly includes a solar cell module, acontroller, and an inverter. The photovoltaic power system constitutesan important part of national electricity supply. At present, a processof connecting the photovoltaic power system to a power grid is:connecting the photovoltaic power system to the power grid after adirect current generated by the solar cell module is converted by agrid-tie inverter into an alternating current satisfying a requirement.Compared with a central inverter, a photovoltaic string inverterfeatures higher efficiency and better flexibility. Therefore, thephotovoltaic string inverter is more frequently selected to connect thephotovoltaic power system to the power grid.

In the prior art, a photovoltaic virtual synchronous generator (PV-VSG)uses a photovoltaic power unit control system to implement relatedfunctions such as inertia and voltage/reactive power adjustment, byretaining an active-power reserve or configuring an energy storageelement. The energy storage element needs to be added to thephotovoltaic string inverter. However, this increases costs of thephotovoltaic power system and requires additional installation space.Therefore, currently, it is an important research focus to use anactive-power reserve to control a PV-VSG without a need to add an energystorage element to a photovoltaic string inverter.

The active-power reserve is an important indicator of a photovoltaicpower system. Precision of the active-power reserve is highly prone toimpact of changes of external factors such as illumination intensity andambient temperature. In the prior art, controlling a PV-VSG by using anactive-power reserve of the photovoltaic power system is implementedmainly in either of two control manners: an active-power reserve basedon variable power point tracking, or spinning reserve capacity trackingbased on a maximum power point. The foregoing two control manners cannotavoid impact of a change of an external factor on the active-powerreserve, and also quite easily cause fluctuation of a direct current busvoltage and alternating current output power that are of thephotovoltaic string inverter. This is unfavorable to control on thevirtual synchronous generator of the photovoltaic string inverter, andaffects a lifespan of the photovoltaic string inverter.

SUMMARY

In view of this, embodiments of this application provide a photovoltaicpower system and a control method thereof, to implement control on avirtual synchronous generator of a photovoltaic string inverter, andprolong a lifespan of the photovoltaic string inverter, without a needto add an energy storage element.

The embodiments of this application provide the following technicalsolutions.

A first aspect of the embodiments of this application provides aphotovoltaic power system, including photovoltaic strings, a controller,a direct current-to-alternating current (DC/AC) inverter circuit, and NDC/DC converter circuits located at a previous stage of the DC/ACinverter circuit, where each DC/DC converter circuit is connected to atleast one photovoltaic string, and a value of N is a positive integergreater than or equal to 2; the controller is connected to all of theDC/AC inverter circuit and the N DC/DC converter circuits, and isconfigured to: perform maximum power point tracking (MPPT) control on nDC/DC converter circuits, determine a first control parameter thatenables the n DC/DC converter circuits to be in a maximum power pointoperating state, and control, based on the first control parameter andan active-power reserve parameter, (N−n) DC/DC converter circuits tooperate in a constant power generation (CPG) mode, where a value of n isa positive integer greater than or equal to 1 and less than or equal toN−1; and the photovoltaic power system is connected to a power gridthrough an output end of the DC/AC inverter circuit.

According to an embodiment, the N DC/DC converter circuits located atthe previous stage of the DC/AC inverter circuit are randomly dividedinto the n DC/DC converter circuits and the (N−n) DC/DC convertercircuits, where MPPT control is performed on the n DC/DC convertercircuits, and CPG control is performed on the (N−n) DC/DC convertercircuits. The N DC/DC converter circuits located at the previous stageof the DC/AC inverter circuit are controlled in the two differentcontrol manners, to implement a fast and accurate power reserve or limitof a photovoltaic string inverter with any illumination intensity andambient temperature, and eliminate fluctuation of a direct current busvoltage and alternating current output power that are of thephotovoltaic string inverter, thereby prolonging a lifespan of thephotovoltaic string inverter. This further implements control on avirtual synchronous generator of the photovoltaic string inverterwithout a need to add an energy storage element.

In a possible design, the controller includes an MPPT controller,configured to: perform MPPT control on the n DC/DC converter circuits;determine the first control parameter that enables the n DC/DC convertercircuits to be in a maximum power point operating apparatus; and obtaina second control parameter based on the first control parameter and theactive-power reserve parameter. The controller further includes a CPGcontroller, which may be configured to perform CPG control on the (N−n)DC/DC converter circuits based on the second control parameter, so thatthe (N−n) DC/DC converter circuits operate in the CPG mode.

According to an embodiment, the controller includes the MPPT controllerand the CPG controller; the MPPT controller is configured to performMPPT control on the n DC/DC converter circuits; and the obtained secondcontrol parameter is used as a reference for the CPG controller toperform CPG control on the (N−n) DC/DC converter circuits. Differentcontrol may be performed on the DC/DC converter circuits located at theprevious stage of the DC/AC inverter circuit, to implement a fast andaccurate power reserve or limit of the photovoltaic string inverter withany illumination intensity and ambient temperature, and eliminatefluctuation of a direct current bus voltage and alternating currentoutput power that are of the photovoltaic string inverter, therebyprolonging a lifespan of the photovoltaic string inverter.

In an embodiment, the controller includes a VSG controller, configuredto calculate a VSG power parameter based on a grid connection parameterfor the power grid and a VSG control algorithm. The controller furtherincludes an MPPT controller, configured to: perform MPPT control on then DC/DC converter circuits; determine the first control parameter thatenables the n DC/DC converter circuits to be in a maximum power pointoperating state; and obtain a second control parameter based on thefirst control parameter, the VSG power parameter, and the active-powerreserve parameter. The controller further includes a CPG controller,configured to perform CPG control on the (N−n) DC/DC converter circuitsbased on the second control parameter, so that the (N−n) DC/DC convertercircuits operate in the CPG mode.

It should be noted that the VSG controller is configured to calculatethe VSG power parameter based on an actually detected current power gridfrequency, a rated power grid frequency, and any virtual inertia of aconstant virtual inertia, an adaptive zero virtual inertia, and anadaptive negative virtual inertia by using the VSG control algorithm,where

the constant virtual inertia is a constant virtual inertia time constantin the VSG control algorithm, the adaptive zero virtual inertia is anadaptive zero virtual inertia time constant in the VSG controlalgorithm, and the adaptive negative virtual inertia is an adaptivenegative virtual inertia time constant in the VSG control algorithm.

According to an embodiment, the controller includes the MPPT controllerand the CPG controller; the MPPT controller is configured to performMPPT control on the n DC/DC converter circuits; and the obtained secondcontrol parameter is used as a reference for the constant powergeneration CPG controller to perform CPG control on the (N−n) DC/DCconverter circuits. In this way, control on the photovoltaic stringinverter is implemented.

It should be noted that based on the foregoing possible designs, theMPPT controller may be constituted in a plurality of structures. In anembodiment, the MPPT controller includes n control circuits, a firstarithmetic unit, and a second arithmetic unit, or alternatively, theMPPT controller includes n control circuits, a first arithmetic unit,and a third arithmetic unit, where each control circuit includes an MPPTprocessing unit and a multiplier. In an embodiment, the MPPT controllerincludes n MPPT processing units, a first arithmetic unit, and a fourtharithmetic unit. By using the plurality of structures disclosed in thisembodiment of this application, the MPPT controller performs MPPTcontrol on the n DC/DC converter circuits; determines the first controlparameter that enables the n DC/DC converter circuits to be in a maximumpower point operating apparatus; and obtains the second controlparameter based on the first control parameter and the active-powerreserve parameter. For a specific implementation process, refer torelated descriptions in this specification.

A second aspect of the embodiments of this application provides aphotovoltaic power system control method, which is applicable to thephotovoltaic power system provided in the first aspect of theembodiments of this application. The control method includes:

performing MPPT control on n DC/DC converter circuits, and determining afirst control parameter that enables the n DC/DC converter circuits tobe in a maximum power point operating state, where a value of n is apositive integer greater than or equal to 1 and less than or equal toN−1; and

controlling, based on the first control parameter and an active-powerreserve parameter, (N−n) DC/DC converter circuits to operate in aconstant power generation CPG mode.

According to the solution, N DC/DC converter circuits are controlled inthe two different control manners, to implement a fast and accuratepower reserve or limit of a photovoltaic string inverter with anyillumination intensity and ambient temperature, and eliminatefluctuation of a direct current bus voltage and alternating currentoutput power that are of the photovoltaic string inverter. In addition,control on a virtual synchronous generator of the photovoltaic stringinverter is implemented, and a lifespan of the photovoltaic stringinverter is prolonged, without a need to add an energy storage element.This further implements control on the virtual synchronous generator ofthe photovoltaic string inverter without a need to add the energystorage element.

In an embodiment, the controlling, based on the first control parameterand an active-power reserve parameter, (N−n) DC/DC converter circuits tooperate in a CPG mode includes:

obtaining a second control parameter based on the first controlparameter and the active-power reserve parameter; and

controlling, based on the second control parameter, the (N−n) DC/DCconverter circuits to operate in the CPG mode.

In an embodiment, the controlling, based on the first control parameterand an active-power reserve parameter, (N−n) DC/DC converter circuits tooperate in the CPG mode includes: obtaining a VSG power parameter basedon a grid connection parameter for a power grid and a VSG controlalgorithm; obtaining a second control parameter based on the firstcontrol parameter, the VSG power parameter, and the active-power reserveparameter; and controlling, based on the second control parameter, the(N−n) DC/DC converter circuits to operate in the CPG mode.

In an embodiment, the obtaining a VSG power parameter based on a gridconnection parameter for a power grid and a VSG control algorithmincludes:

calculating the VSG power parameter based on an actually detectedcurrent power grid frequency, a rated power grid frequency, and any oneof a constant virtual inertia, an adaptive zero virtual inertia, and anadaptive negative virtual inertia by using the VSG control algorithm,where

the constant virtual inertia is a constant virtual inertia time constantin the VSG control algorithm, the adaptive zero virtual inertia is anadaptive zero virtual inertia time constant in the VSG controlalgorithm, and the adaptive negative virtual inertia is an adaptivenegative virtual inertia time constant in the VSG control algorithm.

A third aspect of the embodiments of this application provides acontroller, including a memory and a processor that communicates withthe memory, where

the memory is configured to store program code for controlling aphotovoltaic string inverter; and

the processor is configured to invoke the program code, in the memory,for controlling the photovoltaic string inverter, to perform aphotovoltaic string inverter control method provided in the secondaspect of the embodiments of this application.

A fourth aspect of the embodiments of this application provides anonvolatile computer-readable storage medium, configured to store acomputer program, where the computer program includes an instructionused to perform the method in any possible design of the second aspectof the embodiments of this application.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic structural diagram of a photovoltaic power systemaccording to an embodiment of this application;

FIG. 2 is a schematic structural diagram of a controller according to anembodiment of this application;

FIG. 3 is a schematic diagram of an execution principle of executing anMPPT control algorithm by an MPPT controller according to an embodimentof this application;

FIG. 4 is a schematic diagram of another execution principle ofexecuting an MPPT control algorithm by an MPPT controller according toan embodiment of this application;

FIG. 5 is a schematic diagram of another execution principle ofexecuting an MPPT control algorithm by an MPPT controller according toan embodiment of this application;

FIG. 6 is a schematic structural diagram of another controller accordingto an embodiment of this application;

FIG. 7 is a schematic diagram of an execution principle of executing aVSG control algorithm by a VSG controller according to an embodiment ofthis application;

FIG. 8 is a schematic diagram of an execution principle of executing aVSG control algorithm by a VSG controller according to an embodiment ofthis application;

FIG. 9 is a schematic diagram of an execution principle of executing aVSG control algorithm by a VSG controller according to an embodiment ofthis application;

FIG. 10 is a schematic diagram of an execution principle of executing anMPPT control algorithm by an MPPT controller according to an embodimentof this application;

FIG. 11 is a schematic diagram of another execution principle ofexecuting an MPPT control algorithm by an MPPT controller according toan embodiment of this application;

FIG. 12 is a schematic diagram of another execution principle ofexecuting an MPPT control algorithm by an MPPT controller according toan embodiment of this application;

FIG. 13 is a schematic flowchart of a photovoltaic power system controlmethod according to an embodiment of this application;

FIG. 14 is a schematic flowchart of a control method performed by acontroller according to an embodiment of this application;

FIG. 15 is a schematic flowchart of an execution method for executing anMPPT control algorithm by an MPPT controller according to an embodimentof this application;

FIG. 16 is a schematic flowchart of another execution method forexecuting an MPPT control algorithm by an MPPT controller according toan embodiment of this application;

FIG. 17 is a schematic flowchart of another execution method forexecuting an MPPT control algorithm by an MPPT controller according toan embodiment of this application;

FIG. 18 is a schematic flowchart of another control method performed bycontrollers according to an embodiment of this application;

FIG. 19 is a schematic flowchart of an execution method for executing anMPPT control algorithm by an MPPT controller according to an embodimentof this application;

FIG. 20 is a schematic flowchart of another execution method forexecuting an MPPT control algorithm by an MPPT controller according toan embodiment of this application;

FIG. 21 is a schematic flowchart of another execution method forexecuting an MPPT control algorithm by an MPPT controller according toan embodiment of this application; and

FIG. 22 is a schematic structural diagram of a controller according toan embodiment of this application.

DESCRIPTION OF EMBODIMENTS

The following describes the technical solutions in the embodiments ofthis application with reference to the accompanying drawings in theembodiments of this application. In description of this application, “/”means “or” unless otherwise specified. For example, AB may represent Aor B. In this specification, “and/or” describes only an associationrelationship for describing associated objects and represents that threerelationships may exist. For example, A and/or B may represent thefollowing three cases: Only A exists, both A and B exist, and only Bexists. In addition, in the descriptions of this application, “aplurality of” means two or more than two. In addition, for cleardescription of the technical solutions of the embodiments of thisapplication, in the embodiments of this application, terms such as“first” and “second” are used to distinguish between same or similarobjects having a basically same function and effect. A person skilled inthe art can understand that the terms such as “first” and “second” arenot used to limit a quantity and an execution sequence, and that theterms such as “first” and “second” are unnecessarily limited to bedifferent.

Moreover, terms “include” and “have” in the embodiments of thisapplication, the claims, and the accompanying drawings are inclusive.For example, a process, a method, a system, a product, or a deviceincluding a series of operations or units is not limited to the listedoperations or units, and may further include operations or units thatare not listed.

A photovoltaic power system is a power generation system includingdevices such as photovoltaic modules, an inverter, a cable, and atransformer, and can convert solar energy into usable electrical energyand output the electrical energy to a power grid or an off-grid system.

The photovoltaic modules are direct current power supplies formed aftersolar cells are connected in series and in parallel and then arepackaged.

In the embodiments of this application, the inverter is a photovoltaicstring inverter. A direct current side of the photovoltaic stringinverter may be connected to a plurality of photovoltaic strings thatare not in a parallel connection. The photovoltaic string inverter mayuse two levels of power conversion: conversion from a direct current toa direct current and conversion from a direct current to an alternatingcurrent.

A photovoltaic string is a direct current power supply formed throughend-to-end series connection of positive and negative electrodes of aplurality of photovoltaic modules.

FIG. 1 is a schematic structural diagram of a photovoltaic power systemaccording to an embodiment of this application. The photovoltaic powersystem includes a photovoltaic string inverter, photovoltaic strings 11,and a controller 12. The photovoltaic string inverter mainly includes aDC/AC inverter circuit 101 and a total of N DC/DC converter circuits,that is, a DC/DC converter circuit 1021 to a DC/DC converter circuit102N. A value of N is a positive integer greater than or equal to 2,that is, N≥2.

The N DC/DC converter circuits are located at a previous stage of theDC/AC inverter circuit 101. Each DC/DC converter circuit is connected toat least one photovoltaic string 11.

In an embodiment, a connection relationship between a DC/DC convertercircuit and a photovoltaic string is as follows:

A positive electrode of an input port of each DC/DC converter circuit isconnected to a positive electrode of a photovoltaic string that is in asame group as the DC/DC converter circuit, and a negative electrode ofthe input port of the DC/DC converter circuit is connected to a negativeelectrode of the photovoltaic string that is in the same group as theDC/DC converter circuit.

Each DC/DC converter circuit and a photovoltaic string connected to theDC/DC converter circuit are considered as being in a same group.Photovoltaic strings in a same group are in a parallel connectionrelationship.

In an embodiment, a connection relationship between the DC/AC invertercircuit 101 and the N DC/DC converter circuits that are located at theprevious stage of the DC/AC inverter circuit 101 is as follows:

A positive electrode of an output port of each DC/DC converter circuitis connected in parallel to a positive electrode of an input port on adirect current side of the DC/AC inverter circuit 101, and a negativeelectrode of the output port of the DC/DC converter circuit is connectedin parallel to a negative electrode of the input port on the directcurrent side of the DC/AC inverter circuit 101.

It should be noted that the string inverter may be applied tophotovoltaic power generation scenarios, such as an application scenarioof a large-sized photovoltaic station, application scenarios of smalland medium-sized distributed power stations, and an application scenarioof a residential photovoltaic power system.

An alternating current cable outlet terminal of the DC/AC invertercircuit 101 is used as an output port of the string inverter, and isconnected to a power grid through a cable. Specifically, the alternatingcurrent cable outlet terminal may be connected to a transformer, or maydirectly be connected to a single-phase or three-phase alternatingcurrent power grid.

The controller 13 is connected to the DC/AC inverter circuit 101 and theN DC/DC converter circuits.

In an embodiment of this application, the controller 13 is configuredto: perform maximum power point tracking (MPPT) control on n DC/DCconverter circuits, determine a first control parameter that enables then DC/DC converter circuits to be in a maximum power point operatingstate, and control, based on the first control parameter and anactive-power reserve parameter, (N−n) DC/DC converter circuits tooperate in a constant power generation (CPG) mode, where a value of n isa positive integer greater than or equal to 1 and less than or equal toN−1, that is, 1≤n≤N−1.

In an embodiment, the N DC/DC converter circuits are randomly dividedinto the n DC/DC converter circuits and the (N−n) DC/DC convertercircuits in advance, provided that N≥2 and 1≤n≤N−1 are satisfied.

In an embodiment, the controller 13 is further configured to: collectparameters such as an input voltage and an input current that are ofeach DC/DC converter circuit, a direct current bus voltage, and analternating current power grid voltage and an alternating current outputcurrent that are of the DC/AC inverter circuit in real time; provide apulse-width modulation (PWM) control signal of each DC/DC convertercircuit in real time according to a control policy of the DC/DCconverter circuit; and provide a PWM control signal of the DC/ACinverter circuit in real time according to a control policy of the DC/ACinverter circuit.

In the photovoltaic power system disclosed in an embodiment of thisapplication, master control is performed on the n DC/DC convertercircuits in the N DC/DC converter circuits located at the previous stageof the DC/AC inverter circuit, that is, MPPT control is performed, sothat the n DC/DC converter circuits operate in an MPPT mode; and slavecontrol is performed on the (N−n) DC/DC converter circuits, that is, CPGcontrol is performed, so that the (N−n) DC/DC converter circuits operatein a CPG mode. In this embodiment of this application, master-slavecontrol is implemented on the N DC/DC converter circuits located at theprevious stage of the DC/AC inverter circuit, to reduce impact ofillumination intensity and ambient temperature on an active-powerreserve of the photovoltaic string inverter, implement a fast andaccurate power reserve or limit of the photovoltaic string inverter withany illumination intensity and ambient temperature, and eliminatefluctuation of a direct current bus voltage and alternating currentoutput power that are of the photovoltaic string inverter in a controlprocess. Further, control on a virtual synchronous generator of thephotovoltaic string inverter is implemented, and a lifespan of thephotovoltaic string inverter is prolonged, without a need to add anenergy storage element.

In the photovoltaic power system shown in FIG. 1, there may be aplurality of control manners in which the controller 13 performs MPPTcontrol on then DC/DC converter circuits in the N DC/DC convertercircuits and performs CPG control on the (N−n) DC/DC converter circuitsin the N DC/DC converter circuits. This embodiment of this applicationprovides detailed descriptions by using the following embodiments.

FIG. 2 is a schematic structural diagram of a controller 13 according toan embodiment of this application. The controller 13 includes an MPPTcontroller 201 and a CPG controller 202.

The MPPT controller 201 is configured to: perform MPPT control on nDC/DC converter circuits; determine a first control parameter thatenables the n DC/DC converter circuits to be in a maximum power pointoperating apparatus; and obtain a second control parameter based on thefirst control parameter and an active-power reserve parameter.

The n DC/DC converter circuits are master-controlled DC/DC convertercircuits.

The CPG controller 202 is configured to perform CPG control on (N−n)DC/DC converter circuits based on the second control parameter, so thatthe (N−n) DC/DC converter circuits operate in a constant powergeneration CPG mode.

The (N−n) DC/DC converter circuits are slave-controlled DC/DC convertercircuits.

In specific implementation, there are a plurality of manners in whichthe MPPT controller 201 performs MPPT control on the n DC/DC convertercircuits.

FIG. 3 is a schematic diagram of an execution principle of executing anMPPT control algorithm by an MPPT controller according to an embodimentof this application. The MPPT controller 201 includes a total of ncontrol circuits, that is, a control circuit 3011 to a control circuit301 n, a first arithmetic unit 302, and a second arithmetic unit 303.

Each control circuit includes an MPPT processing unit and a multiplier.

The MPPT processing unit is configured to: detect a first input voltageand a first input current of a corresponding DC/DC converter circuit;determine current input power of the corresponding DC/DC convertercircuit based on the first input voltage and the first input current;and obtain a second input current at a maximum power point of the DC/DCconverter circuit by using an MPPT control algorithm.

The multiplier is connected to the maximum power point tracking MPPTprocessing unit, and is configured to calculate a product of the firstinput voltage and the second input current to obtain a maximum outputactive-power parameter.

The first arithmetic unit 302 connected to the n control circuits isconfigured to: determine a maximum output active-power parameter outputby each control circuit; perform summation and averaging operations on nmaximum output active-power parameters; and use an obtained maximumoutput active-power average value as a first control parameter.

The second arithmetic unit 303 is configured to: determine a powerparameter based on an active-power reserve parameter and the firstcontrol parameter; and use the power parameter as a second controlparameter.

Correspondingly,

a CPG controller is configured to control, based on the second controlparameter, (N−n) DC/DC converter circuits to operate in a poweradjustment-based constant power generation CPG mode.

The first control circuit and the n^(th) control circuit shown in FIG. 3are used as an example for description, and a same processing manner isalso applied to other middle control circuits.

For the first control circuit, the MPPT processing unit is configuredto: detect a first input voltage v_(pv1) and a first input currenti_(pv1) of a first master-controlled DC/DC converter, and obtain aninput current i_(MPPT1) at a maximum power point through MPPT control;and

the multiplier is configured to perform a multiplication operation onthe first input voltage v_(pv1) of the first master-controlled DC/DCconverter and the second input current i_(MPPT1) at the maximum powerpoint, to obtain a maximum output active-power parameter P_(MPPT1).

For the n^(th) control circuit, the MPPT processing unit is configuredto: detect a first input voltage v_(pvn) and a first input currenti_(pvn) of the first master-controlled DC/DC converter, and obtain aninput current i_(MPPTn) at a maximum power point through MPPT control;and

the multiplier is configured to perform a multiplication operation onthe first input voltage v_(pvn) of the first master-controlled DC/DCconverter and the second input current i_(MPPTn) at the maximum powerpoint, to obtain a maximum output active-power parameter P_(MPPTn).

The first arithmetic unit 302 is configured to: perform summation onobtained maximum output active-power parameters P_((MPPT1-MPPTn)) of nmaster-controlled DC/DC converters; perform an averaging operation on avalue obtained through summation; and use an obtained maximum outputactive-power average value as an active-power reference parameterP_(ref1) of (N−n) slave-controlled DC/DC converters.

The second arithmetic unit 303 is configured to: calculate theactive-power reference parameter P_(ref1) and an active-power reserveparameter ΔP according to formula (1); and use an obtained powerparameter as a second control parameter P_(ref), whereP _(ref) =P _(ref1) −ΔP  (1)

The active-power reserve parameter ΔP is an active-power reserve/limitparameter ΔP.

The power parameter P_(ref) calculated according to formula (1) is usedas a power parameter for CPG control on the (N−n) DC/DC convertercircuits.

Correspondingly, the CPG controller 202 shown in FIG. 2 is configured tocontrol, based on the power parameter P_(ref) for CPG control obtainedby the MPPT processing unit, the (N−n) DC/DC converter circuits tooperate in a power adjustment-based CPG (P-CPG) mode, to obtain a PWMsignal corresponding to each DC/DC converter circuit.

The PWM control signal is used as a modulation signal for driving anaction of a switching transistor.

In an embodiment, the CPG controller 202 compares the obtained powerparameter P_(ref) with output active-power P_(pv_m) of an m^(th)slave-controlled DC/DC converter, and a proportional integral PIcontroller obtains a PWM_(m) control signal of the m^(th)slave-controlled DC/DC converter based on an obtained power comparisonresult, where m=n+1, n+2, . . . , N.

It should be noted that, a control manner in which the (N−n) DC/DCconverter circuits are controlled to operate in a power adjustment-basedCPG (P-CPG) mode may be a power control manner such as proportionalintegral control, direct power control, and model prediction control.Details are not described in this embodiment of this application.

A photovoltaic string inverter disclosed in this embodiment of thisapplication does not need to be provided with a solar radiant intensitydetection apparatus. Therefore, costs of the photovoltaic stringinverter can be reduced. Moreover, master MPPT control is performed on nDC/DC converter circuits in N DC/DC converter circuits located at aprevious stage of a DC/AC inverter circuit, and slave CPG control isperformed on the (N−n) DC/DC converter circuits, to implement a fast andaccurate power reserve or limit of the photovoltaic string inverter withany illumination intensity and ambient temperature, and eliminatefluctuation of a direct current bus voltage and alternating currentoutput power that are of the photovoltaic string inverter in a controlprocess. In addition, control on a virtual synchronous generator of thephotovoltaic string inverter is implemented, and a lifespan of thephotovoltaic string inverter is prolonged, without a need to add anenergy storage element.

FIG. 4 is a schematic diagram of another execution principle ofexecuting an MPPT control algorithm by an MPPT controller 201 accordingto an embodiment of this application. The MPPT controller 201 includes atotal of n control circuits, that is, a control circuit 4011 to acontrol circuit 401 n, a first arithmetic unit 402, and a thirdarithmetic unit 403.

Each control circuit includes an MPPT processing unit and a multiplier.

The MPPT processing unit is configured to: detect a first input voltageand a first input current of a corresponding DC/DC converter circuit;determine current input power of the corresponding DC/DC convertercircuit based on the first input voltage and the first input current;and obtain a second input current at a maximum power point of the DC/DCconverter circuit by using an MPPT control algorithm.

The second input current at the maximum power point is obtained based onthe MPPT control algorithm.

The multiplier is connected to the MPPT processing unit, and isconfigured to calculate a product of the first input voltage and thesecond input current to obtain a maximum output active-power parameter.

The first arithmetic unit 402 connected to the n control circuits isconfigured to determine, as a first control parameter, an average valueof maximum output active-power output by the control circuits.

The third arithmetic unit 403 is configured to: determine a currentparameter based on an active-power reserve parameter and the firstcontrol parameter; and use the current parameter as a second controlparameter.

Correspondingly, a CPG controller is configured to control, based on thesecond control parameter, (N−n) DC/DC converter circuits to operate in acurrent adjustment-based constant power generation CPG mode.

The first control circuit and the n^(th) control circuit shown in FIG. 4are used as an example for description, and a same processing manner isalso applied to other middle control circuits.

For the first control circuit, the MPPT processing unit is configuredto: detect a first input voltage v_(pv1) and a first input currenti_(pv1) of a first master-controlled DC/DC converter, and obtain aninput current i_(MPPT1) at a maximum power point through MPPT control;and the multiplier is configured to perform a multiplication operationon the first input voltage v_(pv1) of the first master-controlled DC/DCconverter and the second input current i_(MPPT1) at the maximum powerpoint, to obtain maximum output active-power P_(MPPT1).

For the n^(th) control circuit, the MPPT processing unit is configuredto: detect a first input voltage v_(pvn) and a first input currenti_(pvn) of the first master-controlled DC/DC converter, and obtain aninput current i_(MPPTn) at a maximum power point through MPPT control;and the multiplier is configured to perform a multiplication operationon the first input voltage v_(pvn) of the first master-controlled DC/DCconverter and the second input current i_(MPPTn) at the maximum powerpoint, to obtain a maximum output active-power parameter P_(MPPTn).

The first arithmetic unit 402 is configured to: perform summation onobtained maximum output active-power parameters P_((MPPT1-MPPTn)) of nmaster-controlled DC/DC converters; perform an averaging operation on avalue obtained through summation; and use an obtained maximum outputactive-power average value as an active-power reference P_(ref1) of(N−n) slave-controlled DC/DC converters.

The third arithmetic unit 403 is configured to: calculate theactive-power reference P_(ref1) and an active-power reserve parameter ΔPaccording to formula (1), to obtain a power parameter P_(ref); processthe power parameter P_(ref) according to formula (2); and use anobtained current parameter I_(ref) as a second control parameter, whereI _(ref) =P _(ref) /v _(pv)  (2)

v_(pv) is an input voltage of a slave-controlled DC/DC converter, thatis, a direct-current voltage of a photovoltaic string.

The current parameter I_(ref) calculated according to formula (2) isused as a current instruction for CPG control on the (N−n) DC/DCconverter circuits.

Correspondingly, the CPG controller 202 shown in FIG. 2 is configured tocontrol, based on the current instruction I_(ref) for CPG controlobtained by the MPPT processing unit, the (N−n) DC/DC converter circuitsto operate in a current adjustment-based CPG (I-CPG) mode, to obtain aPWM signal corresponding to each DC/DC converter circuit.

The PWM control signal is used as a modulation signal for driving anaction of a switching transistor.

In an embodiment, the CPG controller 202 compares the obtained currentinstruction I_(ref) with an input current I_(pv_m) of an m^(th)slave-controlled DC/DC converter, and a proportional integral PIcontroller obtains a PWM_(m) control signal of the m^(th)slave-controlled DC/DC converter based on an obtained current comparisonresult, where m=n+1, n+2, . . . , N.

It should be noted that, a control manner in which the (N−n) DC/DCconverter circuits are controlled to operate in a power adjustment-basedCPG (I-CPG) mode may be a power control manner such as proportionalintegral control, direct power control, and model prediction control.Details are not described in this embodiment of this application.

A photovoltaic string inverter disclosed in an embodiment of thisapplication does not need to be provided with a solar radiant intensitydetection apparatus. Therefore, costs of the photovoltaic stringinverter can be reduced. Moreover, master MPPT control is performed on nDC/DC converter circuits in N DC/DC converter circuits located at aprevious stage of a DC/AC inverter circuit, and slave CPG control isperformed on the (N−n) DC/DC converter circuits, to implement a fast andaccurate power reserve or limit of the photovoltaic string inverter withany illumination intensity and ambient temperature, and eliminatefluctuation of a direct current bus voltage and alternating currentoutput power that are of the photovoltaic string inverter in a controlprocess. In addition, control on a virtual synchronous generator of thephotovoltaic string inverter is implemented, and a lifespan of thephotovoltaic string inverter is prolonged, without a need to add anenergy storage element.

FIG. 5 is a schematic diagram of another execution principle ofexecuting an MPPT control algorithm by an MPPT controller according toan embodiment of this application. The MPPT controller 201 includes atotal of n MPPT processing units, that is, an MPPT processing unit 5011to an MPPT processing unit 501 n, a first arithmetic unit 502, and afourth arithmetic unit 503.

Each MPPT processing unit is configured to: detect a first input voltageand a first input current of a corresponding DC/DC converter circuit;determine current input power of the corresponding DC/DC convertercircuit based on the first input voltage and the first input current;and obtain a second input voltage at a maximum power point of the DC/DCconverter circuit by using a maximum power point tracking MPPT controlalgorithm.

The first arithmetic unit 502 connected to the n MPPT processing unitsis configured to determine a second input voltage output by each MPPTprocessing unit; perform summation and averaging operations on n secondinput voltages; and use an obtained second input voltage average valueas a first control parameter.

The fourth arithmetic unit 503 is configured to: perform calculationbased on an active-power reserve parameter and the first controlparameter; and use an obtained voltage parameter as a second controlparameter.

Correspondingly,

a CPG controller is configured to control, based on the second controlparameter, (N−n) DC/DC converter circuits to operate in a voltageadjustment-based constant power generation CPG mode.

The MPPT processing unit 5011 and the MPPT processing unit 501 n shownin FIG. 5 are used as an example for description, and a same processingmanner is also applied to other MPPT processing units.

For the MPPT processing unit 5011, the MPPT processing unit 5011 isconfigured to: detect a first input voltage v_(pv1) and a first inputcurrent i_(pv1) of a first master-controlled DC/DC converter, and obtainan input voltage v_(MPPT1) at a maximum power point through MPPTcontrol.

For the MPPT processing unit 501 n, the MPPT processing unit 501 n isconfigured to: detect a first input voltage v_(pvn) and a first inputcurrent i_(pvn) of an n^(th) master-controlled DC/DC converter, andobtain an input voltage v_(MPPTn) at a maximum power point through MPPTcontrol.

The first arithmetic unit 502 is configured to: perform summation onobtained input voltages

_((MPPT1-MPPTn)) of n master-controlled DC/DC converters; perform anaveraging operation on a value obtained through summation; and use anobtained input voltage average value as a voltage reference V_(ref1) of(N−n) slave-controlled DC/DC converters.

The fourth arithmetic unit 503 is configured to: process an active-powerreserve parameter ΔP and the voltage reference V_(ref1) according toformula (3); and use an obtained voltage parameter V_(ref) as a secondcontrol parameter, whereV _(ref) =V _(ref1) +ΔP/V _(ref1)  (3)

The voltage parameter V_(ref) calculated according to formula (3) isused as a voltage instruction for CPG control on the (N−n) DC/DCconverter circuits.

Correspondingly, the CPG controller 202 shown in FIG. 2 is configured tocontrol, based on the voltage parameter V_(ref) for CPG control obtainedby the MPPT processing unit, the (N−n) DC/DC converter circuits tooperate in a voltage adjustment-based CPG (V-CPG) mode, to obtain a PWMsignal corresponding to each DC/DC converter circuit.

The PWM control signal is used as a modulation signal for driving anaction of a switching transistor.

In an embodiment, the CPG controller 202 compares the obtained voltageparameter V_(ref) with an input current v_(pv_m) of an m^(th)slave-controlled DC/DC converter, and a proportional integral PIcontroller obtains a PWM_(m) control signal of the m^(th)slave-controlled DC/DC converter based on an obtained voltage comparisonresult, where m=n+1, n+2, . . . , N.

A photovoltaic string inverter disclosed in an embodiment of thisapplication does not need to be provided with a solar radiant intensitydetection apparatus. Therefore, costs of the photovoltaic stringinverter can be reduced. Moreover, master MPPT control is performed on nDC/DC converter circuits in N DC/DC converter circuits located at aprevious stage of a DC/AC inverter circuit, and slave CPG control isperformed on the (N−n) DC/DC converter circuits, to implement a fast andaccurate power reserve or limit of the photovoltaic string inverter withany illumination intensity and ambient temperature, and eliminatefluctuation of a direct current bus voltage and alternating currentoutput power that are of the photovoltaic string inverter in a controlprocess. In addition, control on a virtual synchronous generator of thephotovoltaic string inverter is implemented, and a lifespan of thephotovoltaic string inverter is prolonged, without a need to add anenergy storage element.

As disclosed in an embodiment of this application, master MPPT controlis performed on the n DC/DC converter circuits in the N DC/DC convertercircuits located at the previous stage of the DC/AC inverter circuit,and slave CPG control is performed on the (N−n) DC/DC convertercircuits. In this way, based on master-slave control on the N DC/DCconverters located at the previous stage of the DC/AC inverter circuit,PV-VSG control by using an active-power reserve can be implemented;problems of deterioration in PV-VSG output performance and even systeminstability that are caused by illumination intensity and ambienttemperature changes can be resolved; and an inertia support capabilityof a photovoltaic power system can be enhanced, and fluctuation of thedirect current bus voltage and the output power that are of the stringinverter in the power control process can be eliminated.

Based on the photovoltaic string inverter shown in FIG. 1 in theforegoing embodiment of this application, in the process in which PV-VSGcontrol by using the active-power reserve is implemented based onmaster-slave control on the N DC/DC converters located at the previousstage of the DC/AC inverter circuit, there may be a plurality of controlmanners in which a controller 103 performs MPPT control on the n DC/DCconverter circuits in the N DC/DC converter circuits and performs CPGcontrol on the (N−n) DC/DC converter circuits in the N DC/DC convertercircuits. This embodiment of this application provides detaileddescriptions by using the following embodiments.

FIG. 6 is a schematic structural diagram of another controller 13according to an embodiment of this application. The controller 13includes a VSG controller 601, an MPPT controller 602, and a CPGcontroller 603.

The VSG controller 601 is configured to calculate a VSG power parameterbased on a grid connection parameter for a power grid and a VSG controlalgorithm.

The MPPT controller 602 is configured to: perform MPPT control on nDC/DC converter circuits; determine a first control parameter thatenables the n DC/DC converter circuits to be in a maximum power pointoperating state; and obtain a second control parameter based on thefirst control parameter, the VSG power parameter, and an active-powerreserve parameter.

The n DC/DC converter circuits are master-controlled DC/DC convertercircuits.

The CPG controller 603 is configured to perform CPG control on (N−n)DC/DC converter circuits based on the second control parameter, so thatthe (N−n) DC/DC converter circuits operate in a constant powergeneration CPG mode.

The (N−n) DC/DC converter circuits are slave-controlled DC/DC convertercircuits.

In an embodiment, there are a plurality of manners in which the VSGcontroller 601 obtains the VSG power parameter based on the power gridparameter and the VSG control algorithm.

FIG. 7 is a schematic diagram of an execution principle of executing aVSG control algorithm by a VSG controller according to an embodiment ofthis application.

The VSG controller 601 is configured to calculate a VSG power parameterbased on an actually detected current power grid frequency, a ratedpower grid frequency, and a constant virtual inertia by using thevirtual synchronous generator VSG control algorithm.

The constant virtual inertia is a constant virtual inertia time constantin the VSG control algorithm.

In an embodiment, the VSG controller 601 performs VSG controlcalculation according to formula (4) based on the power grid frequencyf_(pll) actually detected by a phase-locked loop, the rated power gridfrequency f_(ref), and the constant virtual inertia; and uses anobtained VSG power parameter P_(VSG) as the VSG power parameter. The VSGpower parameter P_(VSG1) is a parameter having the constant virtualinertia, where

$\begin{matrix}{P_{VSG} = {\left\lbrack {{K_{f}\left( {f_{ref} - f_{pll}} \right)} - {T_{j\; 1}\frac{df_{pll}}{dt}}} \right\rbrack\frac{P_{N}}{\left( {N - n} \right)f_{ref}}}} & (4)\end{matrix}$

f_(ref) is the rated power grid frequency, P_(N) is rated power of astring inverter, K_(f) is a primary frequency modulation coefficient,and T_(j1) is a constant inertia time constant.

In the process of performing VSG control calculation according toformula (4), a VSG power reference P_(VSG1) may be obtained; and the VSGpower reference P_(VSG1) is limited to be within a preset range, wherethe range is a range [−0.2P_(N), 0.1P_(N)]. By using the VSG powerparameter P_(VSG) obtained by dividing the VSG power reference P_(VSG1)by (N−n), formula (4) may also be expressed as P_(VSG)=P_(VSG1)/(N−n).

FIG. 8 is a schematic diagram of another execution principle ofexecuting a VSG control algorithm by a VSG controller according to anembodiment of this application.

The VSG controller 601 is configured to calculate a VSG power parameterbased on an actually detected current power grid frequency, a ratedpower grid frequency, and an adaptive zero virtual inertia by using theVSG control algorithm.

The adaptive zero virtual inertia is an adaptive zero virtual inertiatime constant in the VSG control algorithm.

In an embodiment, the VSG controller 601 performs VSG controlcalculation according to formula (5) based on the power grid frequencyf_(pll) actually detected by a phase-locked loop, the rated power gridfrequency f_(ref), and the adaptive zero virtual inertia; and uses anobtained VSG power parameter P_(VSG) as the VSG power parameter. The VSGpower parameter P_(VSG) is a parameter having the adaptive zero virtualinertia, where

$\begin{matrix}{P_{VSG} = {\left\lbrack {{K_{f}\left( {f_{ref} - f_{pll}} \right)} - {T_{j\; 2}\frac{df_{pll}}{dt}}} \right\rbrack\frac{P_{N}}{\left( {N - n} \right)f_{ref}}}} & (5)\end{matrix}$

f_(ref) is the rated power grid frequency, P_(N) is rated power of astring inverter, K_(f) is a primary frequency modulation coefficient,and T_(j2) is an adaptive inertia time constant. T_(j2) satisfiesformula (6), where

$\begin{matrix}{T_{j\; 2} = \left\{ \begin{matrix}T_{J\;\_\; m\; i\; n} & {{{\Delta\; f}} = {{{f_{ref} - f_{pll}}} \leq B}} \\T_{J\;\_\; m\;{ax}} & {{\Delta\; f\frac{df_{pll}}{dt}} < {0\mspace{14mu}{and}\mspace{14mu}{{\Delta\; f}}} > B} \\0 & {{\Delta\; f\frac{df_{pll}}{dt}} > {0\mspace{14mu}{and}\mspace{14mu}{{\Delta\; f}}} > B}\end{matrix} \right.} & (6)\end{matrix}$

Δf=f_(ref)−f_(pll), T_(j_min) is an allowed minimum inertia timeconstant, and T_(j_max) is an allowed maximum inertia time constant.

In the process of performing VSG control calculation according toformula (5), a VSG power reference P_(VSG1) may be obtained; and the VSGpower reference P_(VSG1) is limited to be within a preset range, wherethe range is a range [−0.2P_(N), 0.1P_(N)]. By using the VSG powerparameter P_(VSG) obtained by dividing the VSG power reference P_(VSG1)by (N−n), formula (5) may also be expressed as P_(VG)=P_(VSG1)/(N−n).

FIG. 9 is a schematic diagram of another execution principle ofexecuting a VSG control algorithm by a VSG controller according to anembodiment of this application.

The VSG controller 601 is configured to calculate a VSG power parameterbased on an actually detected current power grid frequency, a ratedpower grid frequency, and an adaptive negative virtual inertia by usingthe VSG control algorithm.

The adaptive negative virtual inertia is an adaptive negative virtualinertia time constant in the VSG control algorithm.

In an embodiment, the VSG controller 601 performs VSG controlcalculation according to formula (7) based on the power grid frequencyf_(pll) actually detected by a phase-locked loop, the rated power gridfrequency f_(ref), and the adaptive negative virtual inertia; and usesan obtained VSG power parameter P_(VSG) as the VSG power parameter. TheVSG power parameter P_(VSG) is a parameter having the adaptive zerovirtual inertia, where

$\begin{matrix}{P_{VSG} = {\left\lbrack {{K_{f}\left( {f_{ref} - f_{pll}} \right)} - {T_{j\; 3}\frac{df_{pll}}{dt}}} \right\rbrack\frac{P_{N}}{\left( {N - n} \right)f_{ref}}}} & (7)\end{matrix}$

f_(ref) is the rated power grid frequency, P_(N) is rated power of astring inverter, K_(f) is a primary frequency modulation coefficient,and T_(j3) is an adaptive inertia time constant. T_(j3) satisfiesformula (8), where

$\begin{matrix}{T_{j\; 3} = \left\{ \begin{matrix}T_{J\;\_\; m\; i\; n} & {{{\Delta\; f}} = {{{f_{ref} - f_{pll}}} \leq B}} \\T_{J\;\_\; m\;{ax}} & {{\Delta\; f\frac{df_{pll}}{dt}} < {0\mspace{14mu}{and}\mspace{14mu}{{\Delta\; f}}} > B} \\{- T_{J\;\_\; m\; a\; x}} & {{\Delta\; f\frac{df_{pll}}{dt}} > {0\mspace{14mu}{and}\mspace{14mu}{{\Delta\; f}}} > B}\end{matrix} \right.} & (8)\end{matrix}$

Δf=f_(ref)−f_(pll), T_(j_min) is an allowed minimum inertia timeconstant, and T_(j_max) is an allowed maximum inertia time constant.

In the process of performing VSG control calculation according toformula (7), a VSG power reference P_(VSG1) may be obtained; and the VSGpower reference P_(VSG1) is limited to be within a preset range, wherethe range is a range [−0.2P_(N), 0.1P_(N)]. By using the VSG powerparameter P_(VSG) obtained by dividing the VSG power reference P_(VSG1)by (N−n), formula (7) may also be expressed as P_(VSG)=P_(VSG1)/(N−n).

In an embodiment of this application, based on the VSG power parametersgenerated in FIG. 7 to FIG. 9, there are a plurality of manners in whichthe MPPT controller 602 performs MPPT control on the n DC/DC convertercircuits. A specific principle of generating a second control parameterby the MPPT controller 602 may be described with reference to contentabout generating the second control parameter by the MPPT controller 201in FIG. 3, FIG. 4, and FIG. 5.

With reference to the embodiment as shown in FIG. 3, FIG. 10 is aschematic diagram of an execution principle of executing an MPPT controlalgorithm by an MPPT controller according to an embodiment of thisapplication. The MPPT controller 602 includes a total of n controlcircuits, that is, a control circuit 3011 to a control circuit 301 n, afirst arithmetic unit 302, and a second arithmetic unit 303.

Each control circuit includes a maximum power point tracking MPPTprocessing unit and a multiplier.

The MPPT processing unit is configured to: detect a first input voltageand a first input current of a corresponding DC/DC converter circuit;determine current input power of the corresponding DC/DC convertercircuit based on the first input voltage and the first input current;and obtain a second input current at a maximum power point of the DC/DCconverter circuit by using an MPPT control algorithm.

The multiplier is connected to the MPPT processing unit, and isconfigured to calculate a product of the first input voltage and thesecond input current to obtain a maximum output active-power parameter.

The first arithmetic unit 302 connected to the n control circuits isconfigured to: determine a maximum output active-power parameter outputby each control circuit; perform summation and averaging operations on nmaximum output active-power parameters; and use an obtained maximumoutput active-power average value as a first control parameter.

Parameters used for performing calculation by the second arithmetic unitshown in FIG. 10 are different from the parameters used for performingcalculation by the second arithmetic unit shown in FIG. 3. The secondarithmetic unit 303 is configured to: determine a power parameter basedon an active-power reserve parameter, the first control parameter, and aVSG power parameter; and use the power parameter as a second controlparameter.

Correspondingly, a CPG controller is configured to control, based on thesecond control parameter, (N−n) DC/DC converter circuits to operate in apower adjustment-based constant power generation CPG mode.

The first control parameter is obtained by executing the MPPT controlalgorithm by the MPPT controller shown in FIG. 3 in the embodiment ofthis application. The VSG power parameter is obtained by executing a VSGcontrol algorithm by the VSG controller shown in FIG. 7 to FIG. 9 in theembodiments of this application.

In an embodiment, the first control circuit and the n^(th) controlcircuit shown in FIG. 10 are used as an example for description, and asame processing manner is also applied to other middle control circuits.

For the first control circuit, the MPPT processing unit is configuredto: detect a first input voltage v_(pv1) and a first input currenti_(pv1) of a first master-controlled DC/DC converter, and obtain aninput current i_(MPPT1) at a maximum power point through MPPT control;and

the multiplier is configured to perform a multiplication operation onthe first input voltage v_(pv1) of the first master-controlled DC/DCconverter and the second input current i_(MPPT1) at the maximum powerpoint, to obtain a maximum output active-power parameter P_(MPPT1).

For the n^(th) control circuit, the MPPT processing unit is configuredto: detect a first input voltage v_(pvn) and a first input currenti_(pvn) of the first master-controlled DC/DC converter, and obtain aninput current i_(MPPTn) at a maximum power point through MPPT control;and

the multiplier is configured to perform a multiplication operation onthe first input voltage v_(pvn) of the first master-controlled DC/DCconverter and the second input current i_(MPPTn) at the maximum powerpoint, to obtain a maximum output active-power parameter P_(MPPTn).

The first arithmetic unit 302 is configured to: perform summation onobtained maximum output active-power parameters P_((MPPT1-MPPTn))master-controlled DC/DC converters; perform an averaging operation on avalue obtained through summation; and use an obtained maximum outputactive-power average value as an active-power reference parameterP_(ref1) of (N−n) slave-controlled DC/DC converters.

The second arithmetic unit 303 is configured to: calculate theactive-power reference parameter P_(ref1), an active-power reserveparameter ΔP, and a VSG power parameter P_(VSG) according to formula(9); and use an obtained power parameter as a first control parameterP_(ref), whereP _(ref) =P _(ref1) −ΔP+P _(VSG)  (9)

The active-power reserve parameter ΔP is an active-power reserve/limitparameter ΔP.

The power parameter P_(ref) calculated according to formula (9) is usedas a power parameter for CPG control on the (N−n) DC/DC convertercircuits.

Correspondingly, the CPG controller 603 shown in FIG. 10 is configuredto control, based on the power parameter P_(ref) for CPG controlobtained by the MPPT processing unit, the (N−n) DC/DC converter circuitsto operate in a power adjustment-based CPG (P-CPG) mode.

A PWM control signal is used as a modulation signal for driving anaction of a switching transistor.

In an embodiment, the CPG controller 603 compares the obtained powerparameter P_(ref) with output active-power P_(pv_m) of an m^(th)slave-controlled DC/DC converter, and a proportional integral PIcontroller obtains a PWM_(m) control signal of the m^(th)slave-controlled DC/DC converter based on an obtained power comparisonresult, where m=n+1, n+2, . . . , N.

It should be noted that, a control manner in which the (N−n) DC/DCconverter circuits are controlled to operate in a power adjustment-basedCPG (P-CPG) mode may be a power control manner such as proportionalintegral control, direct power control, and model prediction control.Details are not described in this embodiment of this application.

A photovoltaic string inverter disclosed in this embodiment of thisapplication does not need to be provided with a solar radiant intensitydetection apparatus. Therefore, costs of the photovoltaic stringinverter can be reduced. Moreover, master MPPT control is performed on nDC/DC converter circuits in N DC/DC converter circuits located at aprevious stage of a DC/AC inverter circuit, and slave CPG control isperformed on the (N−n) DC/DC converter circuits, to implement PV-VSGcontrol by using an active-power reserve, implement a fast and accuratepower reserve or limit of the photovoltaic string inverter with anyillumination intensity and ambient temperature, and eliminatefluctuation of a direct current bus voltage and alternating currentoutput power that are of the photovoltaic string inverter in a controlprocess. In addition, control on a virtual synchronous generator of thephotovoltaic string inverter is implemented, and a lifespan of thephotovoltaic string inverter is prolonged, without a need to add anenergy storage element.

With reference to the embodiment as shown in FIG. 4, FIG. 11 is aschematic diagram of an execution principle of executing an MPPT controlalgorithm by an MPPT controller according to an embodiment of thisapplication. The MPPT controller 602 includes a total of n controlcircuits, that is, a control circuit 4011 to a control circuit 401 n, afirst arithmetic unit 402, and a third arithmetic unit 403.

Each control circuit includes an MPPT processing unit and a multiplier.

The MPPT processing unit is configured to: detect a first input voltageand a first input current of a corresponding DC/DC converter circuit;determine current input power of the corresponding DC/DC convertercircuit based on the first input voltage and the first input current;and obtain a second input current at a maximum power point of the DC/DCconverter circuit by using an MPPT control algorithm.

The multiplier is connected to the MPPT processing unit, and isconfigured to calculate a product of the first input voltage and thesecond input current to obtain a maximum output active-power parameter.

The first arithmetic unit 402 connected to the n control circuits isconfigured to: determine a maximum output active-power parameter outputby each control circuit; perform summation and averaging operations on nmaximum output active-power parameters; and use an obtained maximumoutput active-power average value as a first control parameter.

Parameters used for performing calculation by the third arithmetic unitshown in FIG. 11 are different from the parameters used for performingcalculation by the third arithmetic unit shown in FIG. 4. The thirdarithmetic unit 403 is configured to: determine a current parameterbased on an active-power reserve parameter, the first control parameter,and a VSG power parameter; and use the current parameter as a secondcontrol parameter.

Correspondingly,

a CPG controller is configured to control, based on the second controlparameter, (N−n) DC/DC converter circuits to operate in a currentadjustment-based constant power generation CPG mode.

The second control parameter is obtained by executing the MPPT controlalgorithm by the MPPT controller shown in FIG. 4 in the embodiment ofthis application. The VSG power parameter is obtained by executing a VSGcontrol algorithm by the VSG controller shown in FIG. 7 to FIG. 9 in theembodiments of this application.

In an embodiment, the first control circuit and the n^(th) controlcircuit shown in FIG. 11 are used as an example for description, and asame processing manner is also applied to other middle control circuits.

For the first control circuit, the MPPT processing unit is configuredto: detect a first input voltage v_(pv1) and a first input currenti_(pv1) of a first master-controlled DC/DC converter, and obtain aninput current i_(MPPT1) at a maximum power point through MPPT control;and

the multiplier is configured to perform a multiplication operation onthe first input voltage v_(pv1) of the first master-controlled DC/DCconverter and the second input current i_(MPPT1) at the maximum powerpoint, to obtain a maximum output active-power parameter P_(MPPT1).

For the n^(th) control circuit, the MPPT processing unit is configuredto: detect a first input voltage v_(pvn) and a first input currenti_(pvn) of the first master-controlled DC/DC converter, and obtain aninput current i_(MPPTn) at a maximum power point through MPPT control;and

the multiplier is configured to perform a multiplication operation onthe first input voltage v_(pvn) of the first master-controlled DC/DCconverter and the second input current i_(MPPTn) at the maximum powerpoint, to obtain a maximum output active-power parameter P_(MPPTn).

The first arithmetic unit 402 is configured to: perform summation onobtained maximum output active-power P_((MPPT1-MPPTn)) of nmaster-controlled DC/DC converters; perform an averaging operation on avalue obtained through summation; and use an obtained maximum outputactive-power average value as an active-power reference parameterP_(ref1) of (N−n) slave-controlled DC/DC converters.

The third arithmetic unit 403 is configured to: calculate theactive-power reference parameter P_(ref1), an active-power reserveparameter ΔP, and a VSG power parameter P_(VSG) according to formula(9); process a power parameter P_(ref) according to formula (2); and usean obtained current parameter I_(ref) as a first control parameter.

The current parameter I_(ref) calculated according to formula (2) isused as a current instruction for CPG control on the (N−n) DC/DCconverter circuits.

Correspondingly, the CPG controller 603 shown in FIG. 11 is configuredto control, based on the current instruction I_(ref) for CPG controlobtained by the maximum power point tracking MPPT processing unit, the(N−n) DC/DC converter circuits to operate in a current adjustment-basedCPG (I-CPG) mode.

A PWM control signal is used as a modulation signal for driving anaction of a switching transistor.

In an embodiment, the CPG controller 202 compares the obtained currentinstruction I_(ref) with an input current I_(pv_m) of an m^(th)slave-controlled DC/DC converter, and a proportional integral PIcontroller obtains a PWM_(m) control signal of the m^(th)slave-controlled DC/DC converter based on an obtained current comparisonresult, where m=n+1, n+2, . . . , N.

It should be noted that, a control manner in which the (N−n) DC/DCconverter circuits are controlled to operate in a power adjustment-basedCPG (P-CPG) mode may be a power control manner such as proportionalintegral control, direct power control, and model prediction control.Details are not described in this embodiment of this application.

A photovoltaic string inverter disclosed in this embodiment of thisapplication does not need to be provided with a solar radiant intensitydetection apparatus. Therefore, costs of the photovoltaic stringinverter can be reduced. Moreover, master MPPT control is performed on nDC/DC converter circuits in N DC/DC converter circuits located at aprevious stage of a DC/AC inverter circuit, and slave CPG control isperformed on the (N−n) DC/DC converter circuits, to implement PV-VSGcontrol by using an active-power reserve, implement a fast and accuratepower reserve or limit of the photovoltaic string inverter with anyillumination intensity and ambient temperature, and eliminatefluctuation of a direct current bus voltage and alternating currentoutput power that are of the photovoltaic string inverter in a controlprocess. In addition, control on a virtual synchronous generator of thephotovoltaic string inverter is implemented, and a lifespan of thephotovoltaic string inverter is prolonged, without a need to add anenergy storage element.

With reference to the embodiment as shown in FIG. 5, FIG. 12 is aschematic diagram of another execution principle of executing an MPPTcontrol algorithm by an MPPT controller according to an embodiment ofthis application. The MPPT controller 602 includes a total of n MPPTprocessing units, that is, an MPPT processing unit 5011 to an MPPTprocessing unit 501 n, a first arithmetic unit 502, and a fourtharithmetic unit 503.

Each control circuit includes an MPPT processing unit and a multiplier.

The MPPT processing unit is configured to: detect a first input voltageand a first input current of a corresponding DC/DC converter circuit;determine current input power of the corresponding DC/DC convertercircuit based on the first input voltage and the first input current;and obtain a second input current at a maximum power point of the DC/DCconverter circuit by using an MPPT control algorithm.

The multiplier is connected to the MPPT processing unit, and isconfigured to calculate a product of the first input voltage and thesecond input current to obtain maximum output active-power.

The first arithmetic unit 502 connected to the n MPPT processing unitsis configured to determine a second input voltage output by each MPPTprocessing unit; perform summation and averaging operations on n maximumoutput active-power; and use an obtained maximum output active-poweraverage value as a first control parameter.

Parameters used for performing calculation by the fourth arithmetic unitshown in FIG. 12 are different from the parameters used for performingcalculation by the fourth arithmetic unit shown in FIG. 5. The fourtharithmetic unit 503 is configured to: determine a voltage referencevalue based on an active-power reserve parameter, the first controlparameter, and a VSG power parameter; determine a voltage parameterbased on the voltage reference value and the first control parameter;and use the voltage parameter as a second control parameter.

A CPG controller is configured to control, based on the second controlparameter, (N−n) DC/DC converter circuits to operate in a voltageadjustment-based constant power generation CPG mode.

The first control parameter is obtained by executing the MPPT controlalgorithm by the MPPT controller shown in FIG. 5 in the embodiment ofthis application. The VSG power parameter is obtained by executing a VSGcontrol algorithm by the VSG controller shown in FIG. 7 to FIG. 9 in theembodiments of this application.

In an embodiment, the MPPT processing unit 5011 and the MPPT processingunit 501 n shown in FIG. 12 are used as an example for description, anda same processing manner is also applied to other MPPT processing units.

For the MPPT processing unit 5011, the MPPT processing unit 5011 isconfigured to: detect a first input voltage v_(pv1) and a first inputcurrent i_(pv1) of a first master-controlled DC/DC converter, and obtainan input voltage v_(MPPT1) at a maximum power point through MPPTcontrol.

For the MPPT processing unit 501 n, the MPPT processing unit 1201 n isconfigured to: detect a first input voltage v_(vpn) and a first inputcurrent i_(pvn) of an n^(th) master-controlled DC/DC converter, andobtain an input voltage v_(MPPTn) at a maximum power point through MPPTcontrol.

The first arithmetic unit 502 is configured to: perform summation onobtained input voltages

_((MPPT1-MPPTn)) of n master-controlled DC/DC converters; perform anaveraging operation on a value obtained through summation; and use anobtained input voltage average value as a voltage reference V_(ref1) of(N−n) slave-controlled DC/DC converters.

The fourth arithmetic unit 503 is configured to: calculate anactive-power reserve parameter ΔP and a VSG power parameter P_(VSG)according to formula (10), to obtain a voltage reference value; performcalculation based on the voltage reference value and the voltagereference V_(ref1); and use an obtained voltage parameter as a secondcontrol parameter, whereV _(ref) =V _(ref1)+(ΔP−P _(VSG))/i _(pv)  (10)

The voltage parameter V_(ref) calculated according to formula (10) isused as a voltage instruction for CPG control on the (N−n) DC/DCconverter circuits.

Correspondingly, the CPG controller 603 shown in FIG. 12 is configuredto control, based on the voltage parameter V_(ref) for CPG controlobtained by the MPPT processing unit, the (N−n) DC/DC converter circuitsto operate in a voltage adjustment-based CPG (V-CPG) mode.

In an embodiment, the CPG controller 603 compares the obtained voltageparameter V_(ref) with an input current v_(pv_m) of an m^(th)slave-controlled DC/DC converter, and a proportional integral PIcontroller obtains a PWM_(m) control signal of the m^(th)slave-controlled DC/DC converter based on an obtained voltage comparisonresult, where m=n+1, n+2, . . . , N.

A photovoltaic string inverter disclosed in this embodiment of thisapplication does not need to be provided with a solar radiant intensitydetection apparatus. Therefore, costs of the photovoltaic stringinverter can be reduced. Moreover, master MPPT control is performed on nDC/DC converter circuits in N DC/DC converter circuits located at aprevious stage of a DC/AC inverter circuit, and slave CPG control isperformed on the (N−n) DC/DC converter circuits, to implement PV-VSGcontrol by using an active-power reserve, implement a fast and accuratepower reserve or limit of the photovoltaic string inverter with anyillumination intensity and ambient temperature, and eliminatefluctuation of a direct current bus voltage and alternating currentoutput power that are of the photovoltaic string inverter in a controlprocess. In addition, control on a virtual synchronous generator of thephotovoltaic string inverter is implemented, and a lifespan of thephotovoltaic string inverter is prolonged, without a need to add anenergy storage element.

A DC/AC inverter in the photovoltaic string inverter disclosed in thisembodiment of this application is optional, and may be a stringthree-phase inverter or a string single-phase inverter.

In the photovoltaic string inverter disclosed in an embodiment of thisapplication, master control is performed on the n DC/DC convertercircuits in the N DC/DC converter circuits located at the previous stageof the DC/AC inverter circuit, that is, MPPT control is performed, sothat the n DC/DC converter circuits operate in an MPPT mode; and slavecontrol is performed on the (N−n) DC/DC converter circuits, that is, CPGcontrol is performed, so that the (N−n) DC/DC converter circuits operatein a CPG mode. Through master-slave control, a fast and accurate powerreserve or limit of the photovoltaic string inverter with anyillumination intensity and ambient temperature can be implemented, andfluctuation of the direct current bus voltage and the alternatingcurrent output power that are of the photovoltaic string inverter in thecontrol process can be eliminated. In addition, control on thephotovoltaic string inverter is improved, and the lifespan of thephotovoltaic string inverter is prolonged, without a need to add anenergy storage element. Further, PV-VSG control by using theactive-power reserve can be implemented by using the VSG controlalgorithm.

An embodiment of this application further correspondingly discloses,based on the photovoltaic power system shown in the foregoingaccompanying drawing, a control method for controlling the photovoltaicpower system. The control method is detailed by using the followingembodiments.

FIG. 13 is a schematic flowchart of a control method based on thephotovoltaic power system shown in FIG. 1 according to an embodiment ofthis application. With reference to FIG. 1, the photovoltaic powersystem control method includes the following operations.

S1301: Perform MPPT control on n DC/DC converter circuits in N DC/DCconverter circuits; and determine a first control parameter that enablesthe n DC/DC converter circuits to be in a maximum power point operatingstate.

With reference to FIG. 1, when S1301 is being performed, a controller103 performs MPPT control on the n DC/DC converter circuits in the NDC/DC converter circuits; and determines the first control parameterthat enables the n DC/DC converter circuits to be in a maximum powerpoint operating state. For a specific execution principle, refer to thedescription in FIG. 1. Details are not described herein again.

S1302: Control, based on the first control parameter and an active-powerreserve parameter, (N−n) DC/DC converter circuits to operate in aconstant power generation CPG mode.

With reference to FIG. 1, when S1032 is being performed, the CPGcontroller 202 controls, based on a second control parameter, the (N−n)DC/DC converter circuits to operate in a CPG mode.

In the photovoltaic power system control method disclosed in anembodiment of this application, master control is performed on the nDC/DC converter circuits in the N DC/DC converter circuits located at aprevious stage of a DC/AC inverter circuit, that is, MPPT control isperformed, so that the n DC/DC converter circuits operate in an MPPTmode; and slave control is performed on the (N−n) DC/DC convertercircuits, that is, CPG control is performed, so that the (N−n) DC/DCconverter circuits operate in a CPG mode. In an embodiment of thisapplication, master-slave control is implemented on the N DC/DCconverter circuits located at the previous stage of the DC/AC invertercircuit, to reduce impact of illumination intensity and ambienttemperature on an active-power reserve of a photovoltaic stringinverter, implement a fast and accurate power reserve or limit of thephotovoltaic string inverter with any illumination intensity and ambienttemperature, and eliminate fluctuation of a direct current bus voltageand alternating current output power that are of the photovoltaic stringinverter in a control process. Further, control on a virtual synchronousgenerator of the photovoltaic string inverter is implemented, and alifespan of the photovoltaic string inverter is prolonged, without aneed to add an energy storage element.

FIG. 14 is a schematic flowchart of a control method performed by acontroller according to an embodiment of this application. Withreference to FIG. 2, the method includes the following operations.

S1401: Perform MPPT control on n DC/DC converter circuits in N DC/DCconverter circuits, and obtain a first control parameter.

Optionally, as shown in FIG. 15, a specific implementation process ofS1401 includes the following operations.

S1501: Detect a first input voltage and a first input current of each ofthe n DC/DC converter circuits in the N DC/DC converter circuits;determine current input power of the corresponding DC/DC convertercircuit based on the first input voltage and the first input current;and obtain a second input current at a maximum power point of the DC/DCconverter circuit by using an MPPT control algorithm.

With reference to FIG. 3, S1501 is performed by the MPPT processingunit. For a specific execution principle, refer to the correspondingdescription in FIG. 3. Details are not described herein again.

S1502: Calculate a product of the first input voltage and the secondinput current that are corresponding to each of the n DC/DC convertercircuits, to obtain maximum output active-power parameters of the nDC/DC converter circuits.

With reference to FIG. 3, S1502 is performed by the multiplier. For aspecific execution principle, refer to the corresponding description inFIG. 3. Details are not described herein again.

S1503: Perform summation and averaging operations on the maximum outputactive-power parameters of the n DC/DC converter circuits; and use anobtained maximum output active-power average value as the first controlparameter.

With reference to FIG. 3, S1503 is performed by the first arithmeticunit 302. For a specific execution principle, refer to the correspondingdescription in FIG. 3. Details are not described herein again.

S1504: Determine a power parameter based on an active-power reserveparameter and the first control parameter; and use the power parameteras a second control parameter.

With reference to FIG. 3, S1504 is performed by the second arithmeticunit 303. For a specific execution principle, refer to the correspondingdescription in FIG. 3. Details are not described herein again.

S1402: Obtain the second control parameter based on the first controlparameter and the active-power reserve parameter.

With reference to FIG. 2, S1402 is performed by the MPPT controller 201.For a specific execution principle, refer to the correspondingdescription in FIG. 2. Details are not described herein again.

In a photovoltaic power system control method disclosed in an embodimentof this application, no solar radiant intensity detection apparatusneeds to be provided. Therefore, costs of a photovoltaic string invertercan be reduced. Moreover, master MPPT control is performed on the nDC/DC converter circuits in the N DC/DC converter circuits located at aprevious stage of a DC/AC inverter circuit, and slave CPG control isperformed on (N−n) DC/DC converter circuits, to implement a fast andaccurate power reserve or limit of the photovoltaic string inverter withany illumination intensity and ambient temperature, and eliminatefluctuation of a direct current bus voltage and alternating currentoutput power that are of the photovoltaic string inverter in a controlprocess. Further, control on a virtual synchronous generator of thephotovoltaic string inverter is implemented, and a lifespan of thephotovoltaic string inverter is prolonged, without a need to add anenergy storage element.

In an embodiment, FIG. 16 is a schematic flowchart of another executionmethod for executing an MPPT control algorithm by an MPPT controllerthat is correspondingly disclosed in an embodiment of this applicationbased on FIG. 4. The method includes the following operations.

S1601: Detect a first input voltage and a first input current of each ofn DC/DC converter circuits in N DC/DC converter circuits; determinecurrent input power of the corresponding DC/DC converter circuit basedon the first input voltage and the first input current; and obtain asecond input current at a maximum power point of the DC/DC convertercircuit by using an MPPT control algorithm.

With reference to FIG. 4, S1601 is performed by the MPPT processingunit. For a specific execution principle, refer to the correspondingdescription in FIG. 4. Details are not described herein again.

S1602: Calculate a product of the first input voltage and the secondinput current that are corresponding to each of the n DC/DC convertercircuits, to obtain maximum output active-power parameters of the nDC/DC converter circuits.

With reference to FIG. 4, S1602 is performed by the multiplier. For aspecific execution principle, refer to the corresponding description inFIG. 4. Details are not described herein again.

S1603: Perform summation and averaging operations on the maximum outputactive-power parameters of the n DC/DC converter circuits; and use anobtained maximum output active-power average value as a first controlparameter.

With reference to FIG. 4, S1603 is performed by the first arithmeticunit 402. For a specific execution principle, refer to the correspondingdescription in FIG. 4. Details are not described herein again.

S1604: Determine a current parameter based on an active-power reserveparameter and the first control parameter; and use the current parameteras a second control parameter.

With reference to FIG. 4, S1604 is performed by the third arithmeticunit 403. For a specific execution principle, refer to the correspondingdescription in FIG. 4. Details are not described herein again.

In a photovoltaic power system control method disclosed in an embodimentof this application, no solar radiant intensity detection apparatusneeds to be provided. Therefore, costs of a photovoltaic string invertercan be reduced. Moreover, master MPPT control is performed on the nDC/DC converter circuits in the N DC/DC converter circuits located at aprevious stage of a DC/AC inverter circuit, and slave CPG control isperformed on (N−n) DC/DC converter circuits, to implement a fast andaccurate power reserve or limit of the photovoltaic string inverter withany illumination intensity and ambient temperature, and eliminatefluctuation of a direct current bus voltage and alternating currentoutput power that are of the photovoltaic string inverter in a controlprocess. Further, control on a virtual synchronous generator of thephotovoltaic string inverter is implemented, and a lifespan of thephotovoltaic string inverter is prolonged, without a need to add anenergy storage element.

FIG. 17 is a schematic flowchart of another execution method forexecuting an MPPT control algorithm by an MPPT controller that isdisclosed in an embodiment of the present invention based on FIG. 5. Themethod includes the following operations.

S1701: Detect a first input voltage and a first input current of each ofn DC/DC converter circuits in N DC/DC converter circuits; determinecurrent input power of the corresponding DC/DC converter circuit basedon the first input voltage and the first input current; and obtain asecond input voltage at a maximum power point of the DC/DC convertercircuit by using an MPPT control algorithm.

With reference to FIG. 5, S1701 is performed by the MPPT processingunit. For a specific execution principle, refer to the correspondingdescription in FIG. 5. Details are not described herein again.

S1702: Perform summation and averaging operations on second inputvoltages at maximum power points of the n DC/DC converter circuits; anduse an obtained second input voltage average value as a first controlparameter.

With reference to FIG. 5, S1702 is performed by the first arithmeticunit 502. For a specific execution principle, refer to the correspondingdescription in FIG. 5. Details are not described herein again.

S1703: Perform calculation based on an active-power reserve parameterand the first control parameter; and use an obtained voltage parameteras a second control parameter.

With reference to FIG. 5, S1703 is performed by the fourth arithmeticunit 503. For a specific execution principle, refer to the correspondingdescription in FIG. 5. Details are not described herein again.

In a photovoltaic power system control method disclosed in an embodimentof this application, no solar radiant intensity detection apparatusneeds to be provided. Therefore, costs of a photovoltaic string invertercan be reduced. Moreover, master MPPT control is performed on the nDC/DC converter circuits in the N DC/DC converter circuits located at aprevious stage of a DC/AC inverter circuit, and slave CPG control isperformed on (N−n) DC/DC converter circuits, to implement a fast andaccurate power reserve or limit of the photovoltaic string inverter withany illumination intensity and ambient temperature, and eliminatefluctuation of a direct current bus voltage and alternating currentoutput power that are of the photovoltaic string inverter in a controlprocess. Further, control on a virtual synchronous generator of thephotovoltaic string inverter is implemented, and a lifespan of thephotovoltaic string inverter is prolonged, without a need to add anenergy storage element.

As disclosed in an embodiment of this application, master MPPT controlis performed on the n DC/DC converter circuits in the N DC/DC convertercircuits located at the previous stage of the DC/AC inverter circuit,and slave CPG control is performed on the (N−n) DC/DC convertercircuits. In this way, based on master-slave control on the N DC/DCconverters located at the previous stage of the DC/AC inverter circuit,PV-VSG control by using an active-power reserve can be implemented;problems of deterioration in PV-VSG output performance and even systeminstability that are caused by illumination intensity and ambienttemperature changes can be resolved; and an inertia support capabilityof a photovoltaic power system can be enhanced, and fluctuation of thedirect current bus voltage and the output power that are of the stringinverter in the power control process can be eliminated.

Based on the photovoltaic power system control method shown in FIG. 13,in the process in which PV-VSG control by using the active-power reserveis implemented based on master-slave control on the N DC/DC converterslocated at the previous stage of the DC/AC inverter circuit, there maybe a plurality of control policies for providing, by a controller 103, aPWM control signal of each DC/DC converter circuit in real timeaccording to a control policy of the DC/DC converter circuit. Thisembodiment of this application provides detailed descriptions by usingthe following embodiments.

FIG. 18 is a schematic flowchart of another control method performed bycontrollers that is disclosed in an embodiment of the present inventionin correspondence to FIG. 6. The method includes the followingoperations.

S1801: Calculate a VSG power parameter based on a grid connectionparameter for a power grid and a VSG control algorithm.

With reference to FIG. 6, S1801 is performed by the VSG controller 601.For a specific execution principle, refer to the correspondingdescription in FIG. 6. Details are not described herein again.

S1802: Perform maximum power point tracking MPPT control on n DC/DCconverter circuits in N DC/DC converter circuits; and determine a firstcontrol parameter that enables the n DC/DC converter circuits to be in amaximum power point operating state.

S1803: Obtain a second control parameter based on the first controlparameter, the VSG power parameter, and an active-power reserveparameter.

With reference to FIG. 6, S1802 and S1803 are performed by the MPPTcontroller 602. For a specific execution principle, refer to thecorresponding description in FIG. 6. Details are not described hereinagain.

In an embodiment, there are a plurality of manners in which the VSGcontroller 601 obtains the VSG power parameter based on the power gridparameter and the VSG control algorithm. Three manners are disclosed inthis embodiment of this application, but are not limited thereto.

A first manner is performing VSG control calculation based on anactually detected power grid frequency, a rated power grid frequency,and a constant virtual inertia, to obtain the VSG power parameter.

A second manner is performing VSG control calculation based on anactually detected power grid frequency, a rated power grid frequency,and an adaptive zero virtual inertia, to obtain the VSG power parameter.

A third manner is performing VSG control calculation based on anactually detected power grid frequency, a rated power grid frequency,and an adaptive negative virtual inertia, to obtain the VSG powerparameter.

In an embodiment of this application, based on the VSG power parametersgenerated in the foregoing different manners, there are a plurality ofmanners in which the MPPT controller 602 performs MPPT control on the nDC/DC converter circuits. A specific principle of generating the secondcontrol parameter by the MPPT controller 602 may be described withreference to content about generating the second control parameter bythe MPPT controller 201 in FIG. 15, FIG. 16, and FIG. 17.

With reference to the embodiment as shown in FIG. 10, FIG. 19 is aschematic flowchart of an execution method for executing an MPPT controlalgorithm by an MPPT controller that is disclosed in an embodiment ofthis application in correspondence to FIG. 10. The method includes thefollowing operations.

S1901: Detect a first input voltage and a first input current of each ofn DC/DC converter circuits in N DC/DC converter circuits; determinecurrent input power of the corresponding DC/DC converter circuit basedon the first input voltage and the first input current; and obtain asecond input current at a maximum power point of the DC/DC convertercircuit by using an MPPT control algorithm.

With reference to FIG. 10, S1901 is performed by the MPPT processingunit. For a specific execution principle, refer to the correspondingdescription in FIG. 10. Details are not described herein again.

S1902: Calculate a product of the first input voltage and the secondinput current that are corresponding to each of the n DC/DC convertercircuits, to obtain maximum output active-power parameters of the nDC/DC converter circuits.

With reference to FIG. 10, S1902 is performed by the multiplier. For aspecific execution principle, refer to the corresponding description inFIG. 10. Details are not described herein again.

S1903: Perform summation and averaging operations on the maximum outputactive-power parameters of the n DC/DC converter circuits; and use anobtained maximum output active-power average value as a first controlparameter.

With reference to FIG. 10, S1903 is performed by the first arithmeticunit 302. For a specific execution principle, refer to the correspondingdescription in FIG. 10. Details are not described herein again.

S1904: Determine a power parameter based on an active-power reserveparameter, the first control parameter, and a VSG power parameter; anduse the power parameter as a second control parameter.

With reference to FIG. 10, S1904 is performed by the second arithmeticunit 303. For a specific execution principle, refer to the correspondingdescription in FIG. 10. Details are not described herein again.

Correspondingly, operation S1302 shown in FIG. 13 is specifically:Control, based on the first control parameter and the active-powerreserve parameter, the (N−n) DC/DC converter circuits to operate in apower adjustment-based constant power generation CPG mode.

In a photovoltaic power system control method disclosed in an embodimentof this application, no solar radiant intensity detection apparatusneeds to be provided. Therefore, costs of a photovoltaic string invertercan be reduced. Moreover, master MPPT control is performed on the nDC/DC converter circuits in the N DC/DC converter circuits located at aprevious stage of a DC/AC inverter circuit, and slave CPG control isperformed on the (N−n) DC/DC converter circuits, to implement PV-VSGcontrol by using an active-power reserve, implement a fast and accuratepower reserve or limit of the photovoltaic string inverter with anyillumination intensity and ambient temperature, and eliminatefluctuation of a direct current bus voltage and alternating currentoutput power that are of the photovoltaic string inverter in a controlprocess. Further, control on a virtual synchronous generator of thephotovoltaic string inverter is implemented, and a lifespan of thephotovoltaic string inverter is prolonged, without a need to add anenergy storage element.

With reference to the embodiment as shown in FIG. 11, FIG. 20 is aschematic flowchart of another execution method for executing an MPPTcontrol algorithm by an MPPT controller that is disclosed in anembodiment of this application in correspondence to FIG. 11. The methodincludes the following operations.

S2001: Detect a first input voltage and a first input current of each ofn DC/DC converter circuits in N DC/DC converter circuits; determinecurrent input power of the corresponding DC/DC converter circuit basedon the first input voltage and the first input current; and obtain asecond input current at a maximum power point of the DC/DC convertercircuit by using an MPPT control algorithm.

With reference to FIG. 11, S2001 is performed by the MPPT processingunit. For a specific execution principle, refer to the correspondingdescription in FIG. 11. Details are not described herein again.

S2002: Calculate a product of the first input voltage and the secondinput current that are corresponding to each of the n DC/DC convertercircuits, to obtain maximum output active-power parameters of the nDC/DC converter circuits.

With reference to FIG. 11, S2002 is performed by the multiplier. For aspecific execution principle, refer to the corresponding description inFIG. 11. Details are not described herein again.

S2003: Perform summation and averaging operations on the maximum outputactive-power parameters of the n DC/DC converter circuits; and use anobtained maximum output active-power average value as a first controlparameter.

With reference to FIG. 11, S2003 is performed by the first arithmeticunit 402. For a specific execution principle, refer to the correspondingdescription in FIG. 11. Details are not described herein again.

S2004: Determine a current parameter based on an active-power reserveparameter, the first control parameter, and a VSG power parameter; anduse the current parameter as a second control parameter.

With reference to FIG. 11, S2004 is performed by the third arithmeticunit 403. For a specific execution principle, refer to the correspondingdescription in FIG. 11. Details are not described herein again.

Correspondingly, operation S1302 shown in FIG. 13 is specifically:Control, based on the first control parameter, the (N−n) DC/DC convertercircuits to operate in a current adjustment-based constant powergeneration CPG mode.

In a photovoltaic power system control method disclosed in an embodimentof this application, no solar radiant intensity detection apparatusneeds to be provided. Therefore, costs of a photovoltaic string invertercan be reduced. Moreover, master MPPT control is performed on the nDC/DC converter circuits in the N DC/DC converter circuits located at aprevious stage of a DC/AC inverter circuit, and slave CPG control isperformed on the (N−n) DC/DC converter circuits, to implement PV-VSGcontrol by using an active-power reserve, implement a fast and accuratepower reserve or limit of the photovoltaic string inverter with anyillumination intensity and ambient temperature, and eliminatefluctuation of a direct current bus voltage and alternating currentoutput power that are of the photovoltaic string inverter in a controlprocess. In addition, control on a virtual synchronous generator of thephotovoltaic string inverter is implemented, and a lifespan of thephotovoltaic string inverter is prolonged, without a need to add anenergy storage element.

With reference to the embodiment as shown in FIG. 12, FIG. 21 is aschematic flowchart of another execution method for executing an MPPTcontrol algorithm by an MPPT controller that is disclosed in anembodiment of this application in correspondence to FIG. 12. The methodincludes the following operations.

S2101: Detect a first input voltage and a first input current of each ofn DC/DC converter circuits in N DC/DC converter circuits; determinecurrent input power of the corresponding DC/DC converter circuit basedon the first input voltage and the first input current; and obtain asecond input current at a maximum power point of the DC/DC convertercircuit by using an MPPT control algorithm.

With reference to FIG. 12, S2101 is performed by the MPPT processingunit. For a specific execution principle, refer to the correspondingdescription in FIG. 12. Details are not described herein again.

S2102: Perform summation and averaging operations on second inputvoltages at maximum power points of the n DC/DC converter circuits; anduse an obtained second input voltage average value as a first controlparameter.

With reference to FIG. 12, S2102 is performed by a second arithmeticunit 502. For a specific execution principle, refer to the correspondingdescription in FIG. 12. Details are not described herein again.

S2103: Determine a voltage reference value based on an active-powerreserve parameter, the first control parameter, and a VSG powerparameter; determine the voltage reference value based on the voltagereference value and the first control parameter; and use a voltageparameter as a second control parameter.

With reference to FIG. 12, S2103 is performed by the fourth arithmeticunit 503. For a specific execution principle, refer to the correspondingdescription in FIG. 12. Details are not described herein again.

Correspondingly, operation S1302 shown in FIG. 13 is specifically:Control, based on the first control parameter, the (N−n) DC/DC convertercircuits to operate in a voltage adjustment-based constant powergeneration CPG mode.

In a photovoltaic power system control method disclosed in an embodimentof this application, no solar radiant intensity detection apparatusneeds to be provided. Therefore, costs of a photovoltaic string invertercan be reduced. Moreover, master MPPT control is performed on the nDC/DC converter circuits in the N DC/DC converter circuits located at aprevious stage of a DC/AC inverter circuit, and slave CPG control isperformed on the (N−n) DC/DC converter circuits, to implement PV-VSGcontrol by using an active-power reserve, implement a fast and accuratepower reserve or limit of the photovoltaic string inverter with anyillumination intensity and ambient temperature, and eliminatefluctuation of a direct current bus voltage and alternating currentoutput power that are of the photovoltaic string inverter in a controlprocess. Further, control on a virtual synchronous generator of thephotovoltaic string inverter is implemented, and a lifespan of thephotovoltaic string inverter is prolonged, without a need to add anenergy storage element.

A specific principle and execution process of the operations performedspecific to the photovoltaic string inverter disclosed in thisembodiment of the present invention are the same as those of the controlmethods for the virtual synchronous generator of the photovoltaic stringinverter disclosed in the embodiments of the present invention, andreference may be made to corresponding descriptions of the controlmethods for the virtual synchronous generator of the photovoltaic stringinverter disclosed in the embodiments of the present invention. Detailsare not described herein again.

With reference to the photovoltaic power system control methodsdisclosed in the embodiments of this application, the photovoltaic powersystem control method disclosed in this embodiment of this applicationmay be implemented directly by hardware, a processor executing programcode in a memory, or a combination thereof.

As shown in FIG. 22, a controller 2200 includes: a memory 2201, aprocessor 2202 that communicates with the memory, and a communicationsinterface 2203.

The processor 2201 is coupled to the memory 2202 through a bus, and theprocessor 2201 is coupled to the communications interface 2203 throughthe bus.

The processor 2202 may specifically be a central processing unit (CPU),a network processor (NP), an application-specific integrated circuit(ASIC), or a programmable logic device (PLD). The PLD may be a complexprogrammable logic device (CPLD), a field-programmable gate array(FPGA), or a generic array logic (GAL).

The memory 2201 may specifically be a content-addressable memory (CAM)or a random-access memory (RAM). The CAM may be a ternarycontent-addressable memory (TCAM).

The communications interface 2203 may be a wired interface, for example,a fiber distributed data interface (FDDI) or an Ethernet interface.

The memory 2201 may alternatively be integrated into the processor 2202.If the memory 2201 and the processor 2202 are independent components,the memory 2201 is connected to the processor 2202. For example, thememory 2201 may communicate with the processor 2202 through the bus. Thecommunications interface 2203 may communicate with the processor 2202through the bus, or the communications interface 2203 may be connectedto the processor 2202 directly.

The memory 2201 is configured to store program code for controlling aphotovoltaic string inverter. Optionally, the memory 2201 includes anoperating system and an application program, and is configured to carryan operating program, code, or instruction used for the control methodsfor the virtual synchronous generator of the photovoltaic stringinverter disclosed in the embodiments of this application.

When the processor 2202 or a hardware device needs to perform anoperation related to the control methods for the virtual synchronousgenerator of the photovoltaic string inverter disclosed in theembodiments of this application, the processor 2202 or the hardwaredevice can complete a process in which a base station in the embodimentsof this application performs the corresponding control methods for thevirtual synchronous generator of the photovoltaic string inverter, byinvoking and executing the operating program, code, or instructionstored in the memory 2201. A specific process is: The processor 2202invokes the program code, in the memory 2201, for controlling thephotovoltaic string inverter, to perform the control methods for thevirtual synchronous generator of the photovoltaic string inverter.

It can be understood that operations of a network device such asreceiving/sending in the embodiments, shown in FIG. 13 to FIG. 21,corresponding to the control methods for the virtual synchronousgenerator of the photovoltaic string inverter may be receiving/sendingprocessing implemented by the processor, or may be a receiving/sendingprocess completed by a receiver/transmitter. The receiver and thetransmitter may exist alone, or may be integrated into a transceiver. Ina possible implementation, the base station 2200 may further include atransceiver.

All or some of the foregoing embodiments may be implemented by usingsoftware, hardware, firmware, or any combination thereof. When softwareis used to implement the embodiments, the embodiments may be implementedcompletely or partially in a form of a computer program product. Thecomputer program product includes one or more computer instructions.When the computer program instructions are loaded and executed on thecomputer, the procedure or functions according to the embodiments ofthis application are all or partially generated. The computer may be ageneral-purpose computer, a dedicated computer, a computer network, orother programmable apparatuses. The computer instructions may be storedin a computer-readable storage medium or may be transmitted from acomputer-readable storage medium to another computer-readable storagemedium. For example, the computer instructions may be transmitted from awebsite, computer, server, or data center to another website, computer,server, or data center in a wired (for example, a coaxial cable, anoptical fiber, or a digital subscriber line (DSL)) or wireless (forexample, infrared, radio, or microwave) manner. The computer-readablestorage medium may be any usable medium accessible by a computer, or adata storage device, such as a server or a data center, integrating oneor more usable media. The usable medium may be a magnetic medium (forexample, a floppy disk, a hard disk, or a magnetic tape), an opticalmedium (for example, a DVD), a semiconductor medium (for example, asolid-state drive (SSD)), or the like.

An embodiment of this application further discloses a photovoltaic powersystem. The photovoltaic power system includes the photovoltaic stringinverter shown in FIG. 13 to FIG. 22.

In summary, in the photovoltaic power system and the control methodthereof disclosed in the embodiments of this application, no solarradiant intensity detection apparatus needs to be provided. Therefore,costs of the photovoltaic string inverter can be reduced. Moreover,master MPPT control is performed on the n DC/DC converter circuits inthe N DC/DC converter circuits located at the previous stage of theDC/AC inverter circuit, and slave CPG control is performed on the (N−n)DC/DC converter circuits, to implement PV-VSG control by using theactive-power reserve, implement a fast and accurate power reserve orlimit of the photovoltaic string inverter with any illuminationintensity and ambient temperature, and eliminate fluctuation of thedirect current bus voltage and the alternating current output power thatare of the photovoltaic string inverter in the control process. Further,control on the virtual synchronous generator of the photovoltaic stringinverter is implemented, and the lifespan of the photovoltaic stringinverter is prolonged, without a need to add an energy storage element.

What is claimed is:
 1. A photovoltaic power system, comprising: aplurality of photovoltaic strings; a direct current-to-alternatingcurrent (DC/AC) inverter circuit; N DC/DC converter circuits located ata previous stage of the DC/AC inverter circuit, wherein each DC/DCconverter circuit is connected to at least one of the photovoltaicstrings, and a value of N is a positive integer greater than or equal to2; and a controller connected to each of the DC/AC inverter circuit andthe N DC/DC converter circuits, and is configured to: perform maximumpower point tracking (MPPT) control on n DC/DC converter circuits,determine a first control parameter that enables the n DC/DC convertercircuits to be in a maximum power point operating state, and control,based on the first control parameter and an active-power reserveparameter, N−n DC/DC converter circuits to operate in a constant powergeneration (CPG) mode, wherein a value of n is a positive integergreater than or equal to 1 and less than or equal to N−1; and whereinthe photovoltaic power system is connected to a power grid through anoutput end of the DC/AC inverter circuit.
 2. The photovoltaic powersystem according to claim 1, wherein the controller comprises: an MPPTcontroller configured to: perform the MPPT control on the n DC/DCconverter circuits, determine the first control parameter that enablesthe n DC/DC converter circuits to be in the maximum power pointoperating state, and obtain a second control parameter based on thefirst control parameter and the active-power reserve parameter; and aCPG controller configured to perform CPG control on the N−n DC/DCconverter circuits based on the second control parameter, so that theN−n DC/DC converter circuits operate in the CPG mode.
 3. Thephotovoltaic power system according to claim 1, wherein the controllercomprises: a virtual synchronous generator (VSG) controller configuredto calculate a VSG power parameter based on a grid connection parameterfor the power grid and a VSG control algorithm; an MPPT controller,configured to: perform the MPPT control on the n DC/DC convertercircuits, determine the first control parameter that enables the n DC/DCconverter circuits to be in a maximum power point operating state, andobtain a second control parameter based on the first control parameter,the VSG power parameter, and the active-power reserve parameter; and aCPG controller configured to perform CPG control on the N−n DC/DCconverter circuits based on the second control parameter, so that theN−n DC/DC converter circuits operate in the CPG mode.
 4. Thephotovoltaic power system according to claim 3, wherein the VSGcontroller is configured to calculate the VSG power parameter based onan actually detected current power grid frequency, a rated power gridfrequency, and any virtual inertia of a constant virtual inertia, anadaptive zero virtual inertia, and an adaptive negative virtual inertiaby using the VSG control algorithm, wherein the constant virtual inertiais a constant virtual inertia time constant in the VSG controlalgorithm, the adaptive zero virtual inertia is an adaptive zero virtualinertia time constant in the VSG control algorithm, and the adaptivenegative virtual inertia is an adaptive negative virtual inertia timeconstant in the VSG control algorithm.
 5. The photovoltaic power systemaccording to claim 2, wherein the MPPT controller comprises n controlcircuits, a first arithmetic unit, and a second arithmetic unit; eachcontrol circuit comprises an MPPT processing unit and a multiplier; theMPPT processing unit is configured to: detect a first input voltage anda first input current of a corresponding DC/DC converter circuit,determine current input power of the corresponding DC/DC convertercircuit based on the first input voltage and the first input current,and obtain a second input current at a maximum power point of the DC/DCconverter circuit using an MPPT control algorithm; the multiplier isconnected to the MPPT processing unit, and is configured to calculate aproduct of the first input voltage and the second input current toobtain a maximum output active-power parameter; the first arithmeticunit connected to the n control circuits is configured to: determine amaximum output active-power parameter output by each control circuit,perform summation and averaging operations on n maximum outputactive-power parameters, and use an obtained maximum output active-poweraverage value as the first control parameter; and the second arithmeticunit is configured to: determine a power parameter based on theactive-power reserve parameter and the first control parameter, and usethe power parameter as the second control parameter.
 6. The photovoltaicpower system according to claim 2, wherein the MPPT controller comprisesn control circuits, a first arithmetic unit, and a third arithmeticunit; each control circuit comprises an MPPT processing unit and amultiplier; the MPPT processing unit is configured to: detect a firstinput voltage and a first input current of a corresponding DC/DCconverter circuit, determine current input power of the correspondingDC/DC converter circuit based on the first input voltage and the firstinput current, and obtain a second input current at a maximum powerpoint of the DC/DC converter circuit using an MPPT control algorithm;the multiplier is connected to the MPPT processing unit, and isconfigured to calculate a product of the first input voltage and thesecond input current to obtain a maximum output active-power parameter;the first arithmetic unit connected to the n control circuits isconfigured to: determine a maximum output active-power parameter outputby each control circuit, perform summation and averaging operations on nmaximum output active-power parameters, and use an obtained maximumoutput active-power average value as the first control parameter; andthe third arithmetic unit is configured to: determine a currentparameter based on the active-power reserve parameter and the firstcontrol parameter, and use the current parameter as the second controlparameter.
 7. The photovoltaic power system according to claim 2,wherein the MPPT controller comprises n MPPT processing units, a firstarithmetic unit, and a fourth arithmetic unit; each MPPT processing unitis configured to: detect a first input voltage and a first input currentof a corresponding DC/DC converter circuit, determine current inputpower of the corresponding DC/DC converter circuit based on the firstinput voltage and the first input current, and obtain a second inputvoltage at a maximum power point of the DC/DC converter circuit by usingan MPPT control algorithm; the first arithmetic unit connected to the nMPPT processing units is configured to determine a second input voltageoutput by each MPPT processing unit, perform summation and averagingoperations on n second input voltages, and use an obtained second inputvoltage average value as the first control parameter; and the fourtharithmetic unit is configured to: perform calculation based on theactive-power reserve parameter and the first control parameter, and usean obtained voltage parameter as the second control parameter.
 8. Thephotovoltaic power system according to claim 3, wherein the MPPTcontroller comprises n control circuits, a first arithmetic unit, and asecond arithmetic unit; each control circuit comprises an MPPTprocessing unit and a multiplier; the MPPT processing unit is configuredto: detect a first input voltage and a first input current of acorresponding DC/DC converter circuit, determine current input power ofthe corresponding DC/DC converter circuit based on the first inputvoltage and the first input current, and obtain a second input currentat a maximum power point of the DC/DC converter circuit by using amaximum power point tracking MPPT control algorithm; and the multiplieris connected to the MPPT processing unit, and is configured to calculatea product of the first input voltage and the second input current toobtain a maximum output active-power parameter; the first arithmeticunit connected to the n control circuits is configured to: determine amaximum output active-power parameter output by each control circuit,perform summation and averaging operations on n maximum outputactive-power parameters, and use an obtained maximum output active-poweraverage value as the first control parameter; and the second arithmeticunit is configured to: determine a power parameter based on theactive-power reserve parameter, the first control parameter, and the VSGpower parameter, and use the power parameter as the second controlparameter.
 9. The photovoltaic power system according to claim 3,wherein the MPPT controller comprises n control circuits, a firstarithmetic unit, and a third arithmetic unit; each control circuitcomprises an MPPT processing unit and a multiplier; the MPPT processingunit is configured to: detect a first input voltage and a first inputcurrent of a corresponding DC/DC converter circuit, determine currentinput power of the corresponding DC/DC converter circuit based on thefirst input voltage and the first input current, and obtain a secondinput current at a maximum power point of the DC/DC converter circuit byusing a maximum power point tracking MPPT control algorithm; themultiplier is connected to the MPPT processing unit, and is configuredto calculate a product of the first input voltage and the second inputcurrent to obtain a maximum output active-power parameter; the firstarithmetic unit connected to the n control circuits is configured to:determine a maximum output active-power parameter output by each controlcircuit, perform summation and averaging operations on n maximum outputactive-power parameters, and use an obtained maximum output active-poweraverage value as the first control parameter; and the third arithmeticunit is configured to: determine a current parameter based on theactive-power reserve parameter, the first control parameter, and thevirtual synchronous generator VSG power parameter, and use the currentparameter as the second control parameter.
 10. The photovoltaic powersystem according to claim 3, wherein the MPPT controller comprises nMPPT processing units, a first arithmetic unit, and a fourth arithmeticunit; each MPPT processing unit is configured to: detect a first inputvoltage and a first input current of a corresponding DC/DC convertercircuit, determine current input power of the corresponding DC/DCconverter circuit based on the first input voltage and the first inputcurrent, and obtain a second input voltage at a maximum power point ofthe DC/DC converter circuit by using an MPPT control algorithm; thefirst arithmetic unit connected to the n MPPT processing units isconfigured to determine a second input voltage output by each MPPTprocessing unit, perform summation and averaging operations on n secondinput voltages, and use an obtained second input voltage average valueas the first control parameter; the fourth arithmetic unit is configuredto: determine a voltage reference value based on the active-powerreserve parameter, the first control parameter, and the VSG powerparameter, determine a voltage parameter based on the voltage referencevalue and the first control parameter, and use the voltage parameter asthe second control parameter; and the CPG controller is configured tocontrol, based on the second control parameter, N−n DC/DC convertercircuits to operate in a voltage adjustment-based CPG mode.
 11. Thephotovoltaic power system according to claim 1, wherein the DC/ACinverter circuit comprises a string three-phase inverter or a stringsingle-phase inverter.
 12. A method for controlling a photovoltaic powersystem, wherein the photovoltaic power system comprises a plurality ofphotovoltaic strings, a controller, a direct current-to-alternatingcurrent (DC/AC) inverter circuit, and N DC/DC converter circuits locatedat a previous stage of the DC/AC inverter circuit, wherein each DC/DCconverter circuit is connected to at least one of the photovoltaicstrings, a value of N is a positive integer greater than or equal to 2,the DC/AC inverter circuit is connected to the N DC/DC convertercircuits and the controller, and the photovoltaic power system isconnected to a power grid through an output end of the DC/AC invertercircuit, the method comprising: performing maximum power point tracking(MPPT) control on n DC/DC converter circuits, and determining a firstcontrol parameter that enables the n DC/DC converter circuits to be in amaximum power point operating state, wherein a value of n is a positiveinteger greater than or equal to 1 and less than or equal to N−1; andcontrolling, based on the first control parameter and an active-powerreserve parameter, N−n DC/DC converter circuits to operate in a constantpower generation (CPG) mode.
 13. The method according to claim 12,wherein the controlling, of the N−n DC/DC converter circuits to operatein the CPG mode comprises: obtaining a second control parameter based onthe first control parameter and the active-power reserve parameter; andcontrolling, based on the second control parameter, the N−n DC/DCconverter circuits to operate in the CPG mode.
 14. The method accordingto claim 12, wherein the controlling, of the N−n DC/DC convertercircuits to operate in the CPG mode comprises: obtaining a virtualsynchronous generator (VSG) power parameter based on a grid connectionparameter for the power grid and a VSG control algorithm; obtaining asecond control parameter based on the first control parameter, the VSGpower parameter, and the active-power reserve parameter; andcontrolling, based on the second control parameter, the N−n DC/DCconverter circuits to operate in the CPG mode.
 15. The method accordingto claim 14, wherein the obtaining of the VSG power parameter based onthe grid connection parameter for the power grid and the VSG controlalgorithm comprises: calculating the VSG power parameter based on anactually detected current power grid frequency, a rated power gridfrequency, and any one of a constant virtual inertia, an adaptive zerovirtual inertia, and an adaptive negative virtual inertia using the VSGcontrol algorithm, wherein the constant virtual inertia is a constantvirtual inertia time constant in the VSG control algorithm, the adaptivezero virtual inertia is an adaptive zero virtual inertia time constantin the VSG control algorithm, and the adaptive negative virtual inertiais an adaptive negative virtual inertia time constant in the VSG controlalgorithm.
 16. A controller, comprising: a processor; and a memorycoupled to the processor to store instructions, which when executed bythe processor, cause the processor to perform operations of controllinga photovoltaic power system, wherein the photovoltaic power systemcomprises a plurality of photovoltaic strings, a controller, a directcurrent-to-alternating current (DC/AC) inverter circuit, and N DC/DCconverter circuits located at a previous stage of the DC/AC invertercircuit, wherein each DC/DC converter circuit is connected to at leastone of the photovoltaic strings, a value of N is a positive integergreater than or equal to 2, the DC/AC inverter circuit is connected tothe N DC/DC converter circuits and the controller, and the photovoltaicpower system is connected to a power grid through an output end of theDC/AC inverter circuit, the operations comprising: performing maximumpower point tracking (MPPT) control on n DC/DC converter circuits, anddetermining a first control parameter that enables the n DC/DC convertercircuits to be in a maximum power point operating state, wherein a valueof n is a positive integer greater than or equal to 1 and less than orequal to N−1; and controlling, based on the first control parameter andan active-power reserve parameter, N−n DC/DC converter circuits tooperate in a constant power generation (CPG) mode.
 17. The controlleraccording to claim 16, wherein the controlling of the N−n DC/DCconverter circuits to operate in the CPG mode comprises: obtaining asecond control parameter based on the first control parameter and theactive-power reserve parameter; and controlling, based on the secondcontrol parameter, the N−n DC/DC converter circuits to operate in theCPG mode.
 18. The controller according to claim 16, wherein thecontrolling of the N−n DC/DC converter circuits to operate in the CPGmode comprises: obtaining a virtual synchronous generator (VSG) powerparameter based on a grid connection parameter for a power grid and aVSG control algorithm; obtaining a second control parameter based on thefirst control parameter, the VSG power parameter, and the active-powerreserve parameter; and controlling, based on the second controlparameter, the N−n DC/DC converter circuits to operate in the CPG mode.19. The controller according to claim 18, wherein the obtaining of theVSG power parameter based on the grid connection parameter for the powergrid and the VSG control algorithm comprises: calculating the VSG powerparameter based on an actually detected current power grid frequency, arated power grid frequency, and any one of a constant virtual inertia,an adaptive zero virtual inertia, and an adaptive negative virtualinertia using the VSG control algorithm, wherein the constant virtualinertia is a constant virtual inertia time constant in the VSG controlalgorithm, the adaptive zero virtual inertia is an adaptive zero virtualinertia time constant in the VSG control algorithm, and the adaptivenegative virtual inertia is an adaptive negative virtual inertia timeconstant in the VSG control algorithm.